diff --git a/src/app.config.ts b/src/app.config.ts
index 8060762..8893996 100644
--- a/src/app.config.ts
+++ b/src/app.config.ts
@@ -93,6 +93,20 @@ export const appConfig: AppConfig = {
           pollingInterval: 10000 // 10 seconds for polling updates
         }
       }
+    },
+    expenses: {
+      name: 'expenses',
+      enabled: true,
+      lazy: false,
+      config: {
+        apiConfig: {
+          baseUrl: import.meta.env.VITE_LNBITS_BASE_URL || 'http://localhost:5000',
+          timeout: 30000 // 30 seconds for API requests
+        },
+        defaultCurrency: 'sats',
+        maxExpenseAmount: 1000000, // 1M sats
+        requireDescription: true
+      }
     }
   },
   
diff --git a/src/app.ts b/src/app.ts
index 459283e..6878880 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -16,6 +16,7 @@ import chatModule from './modules/chat'
 import eventsModule from './modules/events'
 import marketModule from './modules/market'
 import walletModule from './modules/wallet'
+import expensesModule from './modules/expenses'
 
 // Root component
 import App from './App.vue'
@@ -43,7 +44,8 @@ export async function createAppInstance() {
     ...chatModule.routes || [],
     ...eventsModule.routes || [],
     ...marketModule.routes || [],
-    ...walletModule.routes || []
+    ...walletModule.routes || [],
+    ...expensesModule.routes || []
   ].filter(Boolean)
 
   // Create router with all routes available immediately
@@ -126,6 +128,13 @@ export async function createAppInstance() {
     )
   }
 
+  // Register expenses module
+  if (appConfig.modules.expenses?.enabled) {
+    moduleRegistrations.push(
+      pluginManager.register(expensesModule, appConfig.modules.expenses)
+    )
+  }
+
   // Wait for all modules to register
   await Promise.all(moduleRegistrations)
 
diff --git a/src/core/di-container.ts b/src/core/di-container.ts
index da71624..0d27524 100644
--- a/src/core/di-container.ts
+++ b/src/core/di-container.ts
@@ -160,6 +160,9 @@ export const SERVICE_TOKENS = {
 
   // Image upload services
   IMAGE_UPLOAD_SERVICE: Symbol('imageUploadService'),
+
+  // Expenses services
+  EXPENSES_API: Symbol('expensesAPI'),
 } as const
 
 // Type-safe injection helpers
diff --git a/src/modules/expenses/components/AccountSelector.vue b/src/modules/expenses/components/AccountSelector.vue
new file mode 100644
index 0000000..ec49ed3
--- /dev/null
+++ b/src/modules/expenses/components/AccountSelector.vue
@@ -0,0 +1,257 @@
+<template>
+  <div class="space-y-4">
+    <!-- Breadcrumb showing current path -->
+    <div v-if="selectedPath.length > 0" class="flex items-center gap-2 text-sm">
+      <Button
+        variant="ghost"
+        size="sm"
+        @click="navigateToRoot"
+        class="h-7 px-2"
+      >
+        <ChevronLeft class="h-4 w-4 mr-1" />
+        Start
+      </Button>
+      <ChevronRight class="h-4 w-4 text-muted-foreground" />
+      <div class="flex items-center gap-2">
+        <span
+          v-for="(node, index) in selectedPath"
+          :key="node.account.id"
+          class="flex items-center gap-2"
+        >
+          <Button
+            variant="ghost"
+            size="sm"
+            @click="navigateToLevel(index)"
+            class="h-7 px-2 text-muted-foreground hover:text-foreground"
+          >
+            {{ getAccountDisplayName(node.account.name) }}
+          </Button>
+          <ChevronRight
+            v-if="index < selectedPath.length - 1"
+            class="h-4 w-4 text-muted-foreground"
+          />
+        </span>
+      </div>
+    </div>
+
+    <!-- Account selection list -->
+    <div class="border border-border rounded-lg bg-card">
+      <!-- Loading state -->
+      <div
+        v-if="isLoading"
+        class="flex items-center justify-center py-12"
+      >
+        <Loader2 class="h-6 w-6 animate-spin text-muted-foreground" />
+        <span class="ml-2 text-sm text-muted-foreground">Loading accounts...</span>
+      </div>
+
+      <!-- Error state -->
+      <div
+        v-else-if="error"
+        class="flex flex-col items-center justify-center py-12 px-4"
+      >
+        <AlertCircle class="h-8 w-8 text-destructive mb-2" />
+        <p class="text-sm text-destructive">{{ error }}</p>
+        <Button
+          variant="outline"
+          size="sm"
+          @click="loadAccounts"
+          class="mt-4"
+        >
+          <RefreshCw class="h-4 w-4 mr-2" />
+          Retry
+        </Button>
+      </div>
+
+      <!-- Account list -->
+      <div v-else-if="currentNodes.length > 0" class="divide-y divide-border">
+        <button
+          v-for="node in currentNodes"
+          :key="node.account.id"
+          @click="selectNode(node)"
+          type="button"
+          class="w-full flex items-center justify-between p-4 hover:bg-muted/50 transition-colors text-left"
+        >
+          <div class="flex items-center gap-3">
+            <Folder
+              v-if="node.account.has_children"
+              class="h-5 w-5 text-primary"
+            />
+            <FileText
+              v-else
+              class="h-5 w-5 text-muted-foreground"
+            />
+            <div>
+              <p class="font-medium text-foreground">
+                {{ getAccountDisplayName(node.account.name) }}
+              </p>
+              <p
+                v-if="node.account.description"
+                class="text-sm text-muted-foreground"
+              >
+                {{ node.account.description }}
+              </p>
+            </div>
+          </div>
+          <div class="flex items-center gap-2">
+            <Badge
+              v-if="!node.account.has_children"
+              variant="outline"
+              class="text-xs"
+            >
+              {{ node.account.account_type }}
+            </Badge>
+            <ChevronRight
+              v-if="node.account.has_children"
+              class="h-5 w-5 text-muted-foreground"
+            />
+          </div>
+        </button>
+      </div>
+
+      <!-- Empty state -->
+      <div
+        v-else
+        class="flex flex-col items-center justify-center py-12 px-4"
+      >
+        <Folder class="h-12 w-12 text-muted-foreground mb-2" />
+        <p class="text-sm text-muted-foreground">No accounts available</p>
+      </div>
+    </div>
+
+    <!-- Selected account display -->
+    <div
+      v-if="selectedAccount"
+      class="flex items-center justify-between p-4 rounded-lg border-2 border-primary bg-primary/5"
+    >
+      <div class="flex items-center gap-3">
+        <Check class="h-5 w-5 text-primary" />
+        <div>
+          <p class="font-medium text-foreground">Selected Account</p>
+          <p class="text-sm text-muted-foreground">{{ selectedAccount.name }}</p>
+        </div>
+      </div>
+      <Badge variant="default">{{ selectedAccount.account_type }}</Badge>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { ref, computed, onMounted } from 'vue'
+import { Button } from '@/components/ui/button'
+import { Badge } from '@/components/ui/badge'
+import {
+  ChevronLeft,
+  ChevronRight,
+  Folder,
+  FileText,
+  Loader2,
+  AlertCircle,
+  RefreshCw,
+  Check
+} from 'lucide-vue-next'
+import { injectService, SERVICE_TOKENS } from '@/core/di-container'
+import type { ExpensesAPI } from '../services/ExpensesAPI'
+import type { Account, AccountNode } from '../types'
+
+interface Props {
+  rootAccount?: string
+  modelValue?: Account | null
+}
+
+interface Emits {
+  (e: 'update:modelValue', value: Account | null): void
+  (e: 'account-selected', value: Account): void
+}
+
+const props = defineProps<Props>()
+const emit = defineEmits<Emits>()
+
+// Inject services
+const expensesAPI = injectService<ExpensesAPI>(SERVICE_TOKENS.EXPENSES_API)
+
+// Component state
+const isLoading = ref(false)
+const error = ref<string | null>(null)
+const accountHierarchy = ref<AccountNode[]>([])
+const selectedPath = ref<AccountNode[]>([])
+const selectedAccount = ref<Account | null>(props.modelValue ?? null)
+
+// Current nodes to display (either root or children of selected node)
+const currentNodes = computed(() => {
+  if (selectedPath.value.length === 0) {
+    return accountHierarchy.value
+  }
+
+  const lastNode = selectedPath.value[selectedPath.value.length - 1]
+  return lastNode.children
+})
+
+/**
+ * Get display name from full account path
+ * e.g., "Expenses:Groceries:Organic" -> "Organic"
+ */
+function getAccountDisplayName(fullName: string): string {
+  const parts = fullName.split(':')
+  return parts[parts.length - 1]
+}
+
+/**
+ * Load accounts from API
+ */
+async function loadAccounts() {
+  isLoading.value = true
+  error.value = null
+
+  try {
+    accountHierarchy.value = await expensesAPI.getAccountHierarchy(
+      props.rootAccount
+    )
+  } catch (err) {
+    error.value = err instanceof Error ? err.message : 'Failed to load accounts'
+    console.error('[AccountSelector] Error loading accounts:', err)
+  } finally {
+    isLoading.value = false
+  }
+}
+
+/**
+ * Handle node selection
+ */
+function selectNode(node: AccountNode) {
+  if (node.account.has_children) {
+    // Navigate into folder
+    selectedPath.value.push(node)
+    selectedAccount.value = null
+    emit('update:modelValue', null)
+  } else {
+    // Select leaf account
+    selectedAccount.value = node.account
+    emit('update:modelValue', node.account)
+    emit('account-selected', node.account)
+  }
+}
+
+/**
+ * Navigate back to root
+ */
+function navigateToRoot() {
+  selectedPath.value = []
+  selectedAccount.value = null
+  emit('update:modelValue', null)
+}
+
+/**
+ * Navigate to specific level in breadcrumb
+ */
+function navigateToLevel(level: number) {
+  selectedPath.value = selectedPath.value.slice(0, level + 1)
+  selectedAccount.value = null
+  emit('update:modelValue', null)
+}
+
+// Load accounts on mount
+onMounted(() => {
+  loadAccounts()
+})
+</script>
diff --git a/src/modules/expenses/components/AddExpense.vue b/src/modules/expenses/components/AddExpense.vue
new file mode 100644
index 0000000..6b24130
--- /dev/null
+++ b/src/modules/expenses/components/AddExpense.vue
@@ -0,0 +1,288 @@
+<template>
+  <div class="bg-card border border-border rounded-lg shadow-lg overflow-hidden">
+    <!-- Header -->
+    <div class="flex items-center justify-between p-4 border-b border-border">
+      <div class="flex items-center gap-2">
+        <DollarSign class="h-5 w-5 text-primary" />
+        <h3 class="font-semibold text-foreground">Add Expense</h3>
+      </div>
+      <Button
+        variant="ghost"
+        size="icon"
+        @click="$emit('close')"
+      >
+        <X class="h-4 w-4" />
+      </Button>
+    </div>
+
+    <!-- Form -->
+    <div class="p-4 space-y-4">
+      <!-- Step indicator -->
+      <div class="flex items-center justify-center gap-2 mb-4">
+        <div
+          :class="[
+            'flex items-center justify-center w-8 h-8 rounded-full font-medium text-sm transition-colors',
+            currentStep === 1
+              ? 'bg-primary text-primary-foreground'
+              : selectedAccount
+              ? 'bg-primary/20 text-primary'
+              : 'bg-muted text-muted-foreground'
+          ]"
+        >
+          1
+        </div>
+        <div class="w-12 h-px bg-border" />
+        <div
+          :class="[
+            'flex items-center justify-center w-8 h-8 rounded-full font-medium text-sm transition-colors',
+            currentStep === 2
+              ? 'bg-primary text-primary-foreground'
+              : 'bg-muted text-muted-foreground'
+          ]"
+        >
+          2
+        </div>
+      </div>
+
+      <!-- Step 1: Account Selection -->
+      <div v-if="currentStep === 1">
+        <p class="text-sm text-muted-foreground mb-4">
+          Select the account for this expense
+        </p>
+        <AccountSelector
+          v-model="selectedAccount"
+          root-account="Expenses"
+          @account-selected="handleAccountSelected"
+        />
+      </div>
+
+      <!-- Step 2: Expense Details -->
+      <div v-if="currentStep === 2">
+        <form @submit="onSubmit" class="space-y-4">
+          <!-- Description -->
+          <FormField v-slot="{ componentField }" name="description">
+            <FormItem>
+              <FormLabel>Description *</FormLabel>
+              <FormControl>
+                <Textarea
+                  placeholder="e.g., Groceries for community dinner"
+                  v-bind="componentField"
+                  rows="3"
+                />
+              </FormControl>
+              <FormDescription>
+                Describe what this expense was for
+              </FormDescription>
+              <FormMessage />
+            </FormItem>
+          </FormField>
+
+          <!-- Amount -->
+          <FormField v-slot="{ componentField }" name="amount">
+            <FormItem>
+              <FormLabel>Amount (sats) *</FormLabel>
+              <FormControl>
+                <Input
+                  type="number"
+                  placeholder="0"
+                  v-bind="componentField"
+                  min="1"
+                  step="1"
+                />
+              </FormControl>
+              <FormDescription>
+                Amount in satoshis
+              </FormDescription>
+              <FormMessage />
+            </FormItem>
+          </FormField>
+
+          <!-- Reference (optional) -->
+          <FormField v-slot="{ componentField }" name="reference">
+            <FormItem>
+              <FormLabel>Reference</FormLabel>
+              <FormControl>
+                <Input
+                  placeholder="e.g., Invoice #123, Receipt #456"
+                  v-bind="componentField"
+                />
+              </FormControl>
+              <FormDescription>
+                Optional reference number or note
+              </FormDescription>
+              <FormMessage />
+            </FormItem>
+          </FormField>
+
+          <!-- Account from equity checkbox -->
+          <FormField v-slot="{ value, handleChange }" name="isEquity">
+            <FormItem>
+              <div class="flex items-center space-x-2">
+                <FormControl>
+                  <Checkbox
+                    :model-value="value"
+                    @update:model-value="handleChange"
+                    :disabled="isSubmitting"
+                  />
+                </FormControl>
+                <div class="space-y-1 leading-none">
+                  <FormLabel>Account from equity</FormLabel>
+                  <FormDescription>
+                    Check if this expense should be accounted from your equity
+                  </FormDescription>
+                </div>
+              </div>
+            </FormItem>
+          </FormField>
+
+          <!-- Selected account info -->
+          <div class="p-3 rounded-lg bg-muted/50">
+            <p class="text-sm font-medium text-foreground mb-1">
+              Account: {{ selectedAccount?.name }}
+            </p>
+            <p
+              v-if="selectedAccount?.description"
+              class="text-xs text-muted-foreground"
+            >
+              {{ selectedAccount.description }}
+            </p>
+          </div>
+
+          <!-- Action buttons -->
+          <div class="flex items-center gap-2 pt-2">
+            <Button
+              type="button"
+              variant="outline"
+              @click="currentStep = 1"
+              :disabled="isSubmitting"
+              class="flex-1"
+            >
+              <ChevronLeft class="h-4 w-4 mr-1" />
+              Back
+            </Button>
+            <Button
+              type="submit"
+              :disabled="isSubmitting || !isFormValid"
+              class="flex-1"
+            >
+              <Loader2
+                v-if="isSubmitting"
+                class="h-4 w-4 mr-2 animate-spin"
+              />
+              <span>{{ isSubmitting ? 'Submitting...' : 'Submit Expense' }}</span>
+            </Button>
+          </div>
+        </form>
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { ref, computed } from 'vue'
+import { useForm } from 'vee-validate'
+import { toTypedSchema } from '@vee-validate/zod'
+import * as z from 'zod'
+import { Button } from '@/components/ui/button'
+import { Input } from '@/components/ui/input'
+import { Textarea } from '@/components/ui/textarea'
+import { Checkbox } from '@/components/ui/checkbox'
+import {
+  FormControl,
+  FormDescription,
+  FormField,
+  FormItem,
+  FormLabel,
+  FormMessage,
+} from '@/components/ui/form'
+import { DollarSign, X, ChevronLeft, Loader2 } from 'lucide-vue-next'
+import { injectService, SERVICE_TOKENS } from '@/core/di-container'
+import type { ExpensesAPI } from '../services/ExpensesAPI'
+import type { Account } from '../types'
+import AccountSelector from './AccountSelector.vue'
+
+interface Emits {
+  (e: 'close'): void
+  (e: 'expense-submitted'): void
+  (e: 'action-complete'): void
+}
+
+const emit = defineEmits<Emits>()
+
+// Inject services
+const expensesAPI = injectService<ExpensesAPI>(SERVICE_TOKENS.EXPENSES_API)
+
+// Component state
+const currentStep = ref(1)
+const selectedAccount = ref<Account | null>(null)
+const isSubmitting = ref(false)
+
+// Form schema
+const formSchema = toTypedSchema(
+  z.object({
+    description: z.string().min(1, 'Description is required').max(500, 'Description too long'),
+    amount: z.coerce.number().min(1, 'Amount must be at least 1 sat'),
+    reference: z.string().max(100, 'Reference too long').optional(),
+    isEquity: z.boolean().default(false)
+  })
+)
+
+// Set up form
+const form = useForm({
+  validationSchema: formSchema,
+  initialValues: {
+    description: '',
+    amount: 0,
+    reference: '',
+    isEquity: false
+  }
+})
+
+const { resetForm, meta } = form
+const isFormValid = computed(() => meta.value.valid)
+
+/**
+ * Handle account selection
+ */
+function handleAccountSelected(account: Account) {
+  selectedAccount.value = account
+  currentStep.value = 2
+}
+
+/**
+ * Submit expense
+ */
+const onSubmit = form.handleSubmit(async (values) => {
+  if (!selectedAccount.value) {
+    console.error('[AddExpense] No account selected')
+    return
+  }
+
+  isSubmitting.value = true
+
+  try {
+    await expensesAPI.submitExpense({
+      description: values.description,
+      amount: values.amount,
+      expense_account: selectedAccount.value.name,
+      is_equity: values.isEquity,
+      user_wallet: '', // Will be filled by backend from auth
+      reference: values.reference,
+      currency: 'sats'
+    })
+
+    // Reset form and close
+    resetForm()
+    selectedAccount.value = null
+    currentStep.value = 1
+
+    emit('expense-submitted')
+    emit('action-complete')
+    emit('close')
+  } catch (error) {
+    console.error('[AddExpense] Error submitting expense:', error)
+  } finally {
+    isSubmitting.value = false
+  }
+})
+</script>
diff --git a/src/modules/expenses/index.ts b/src/modules/expenses/index.ts
new file mode 100644
index 0000000..8e21d18
--- /dev/null
+++ b/src/modules/expenses/index.ts
@@ -0,0 +1,58 @@
+/**
+ * Expenses Module
+ *
+ * Provides expense tracking and submission functionality
+ * integrated with castle LNbits extension.
+ */
+
+import type { App } from 'vue'
+import { markRaw } from 'vue'
+import type { ModulePlugin } from '@/core/types'
+import { container, SERVICE_TOKENS } from '@/core/di-container'
+import { ExpensesAPI } from './services/ExpensesAPI'
+import AddExpense from './components/AddExpense.vue'
+
+export const expensesModule: ModulePlugin = {
+  name: 'expenses',
+  version: '1.0.0',
+  dependencies: ['base'],
+
+  quickActions: [
+    {
+      id: 'add-expense',
+      label: 'Expense',
+      icon: 'DollarSign',
+      component: markRaw(AddExpense),
+      category: 'finance',
+      order: 10,
+      requiresAuth: true
+    }
+  ],
+
+  async install(app: App) {
+    console.log('[Expenses Module] Installing...')
+
+    // 1. Create and register service
+    const expensesAPI = new ExpensesAPI()
+    container.provide(SERVICE_TOKENS.EXPENSES_API, expensesAPI)
+
+    // 2. Initialize service (wait for dependencies)
+    await expensesAPI.initialize({
+      waitForDependencies: true,
+      maxRetries: 3,
+      retryDelay: 1000
+    })
+
+    console.log('[Expenses Module] ExpensesAPI initialized')
+
+    // 3. Register components globally (optional, for use outside quick actions)
+    app.component('AddExpense', AddExpense)
+
+    console.log('[Expenses Module] Installed successfully')
+  }
+}
+
+export default expensesModule
+
+// Export types for use in other modules
+export type { Account, AccountNode, ExpenseEntry, ExpenseEntryRequest } from './types'
diff --git a/src/modules/expenses/services/ExpensesAPI.ts b/src/modules/expenses/services/ExpensesAPI.ts
new file mode 100644
index 0000000..0646bc7
--- /dev/null
+++ b/src/modules/expenses/services/ExpensesAPI.ts
@@ -0,0 +1,248 @@
+/**
+ * API service for castle extension expense operations
+ */
+
+import { BaseService } from '@/core/base/BaseService'
+import type {
+  Account,
+  ExpenseEntryRequest,
+  ExpenseEntry,
+  AccountNode
+} from '../types'
+import { appConfig } from '@/app.config'
+
+export class ExpensesAPI extends BaseService {
+  protected readonly metadata = {
+    name: 'ExpensesAPI',
+    version: '1.0.0',
+    dependencies: ['AuthService', 'ToastService']
+  }
+
+  private get config() {
+    return appConfig.modules.expenses.config
+  }
+
+  private get baseUrl() {
+    return this.config?.apiConfig?.baseUrl || ''
+  }
+
+  protected async onInitialize(): Promise<void> {
+    console.log('[ExpensesAPI] Initialized with config:', {
+      baseUrl: this.baseUrl,
+      timeout: this.config?.apiConfig?.timeout
+    })
+  }
+
+  /**
+   * Get authentication headers
+   */
+  private getHeaders(): HeadersInit {
+    if (!this.authService) {
+      throw new Error('AuthService not available')
+    }
+
+    const walletKey = this.authService.walletKey
+    if (!walletKey) {
+      throw new Error('Wallet key not available. Please log in.')
+    }
+
+    return {
+      'Content-Type': 'application/json',
+      'X-Api-Key': walletKey
+    }
+  }
+
+  /**
+   * Get all accounts from castle
+   *
+   * Note: Currently returns all accounts. Once castle API implements
+   * user permissions, use filter_by_user=true parameter.
+   */
+  async getAccounts(): Promise<Account[]> {
+    try {
+      const response = await fetch(`${this.baseUrl}/api/v1/accounts`, {
+        method: 'GET',
+        headers: this.getHeaders(),
+        signal: AbortSignal.timeout(this.config?.apiConfig?.timeout || 30000)
+      })
+
+      if (!response.ok) {
+        throw new Error(`Failed to fetch accounts: ${response.statusText}`)
+      }
+
+      const accounts = await response.json()
+      return accounts as Account[]
+    } catch (error) {
+      console.error('[ExpensesAPI] Error fetching accounts:', error)
+      if (this.toastService) {
+        this.toastService.error('Failed to load accounts', {
+          description: error instanceof Error ? error.message : 'Unknown error'
+        })
+      }
+      throw error
+    }
+  }
+
+  /**
+   * Get accounts in hierarchical tree structure
+   *
+   * Converts flat account list to nested tree based on colon-separated names
+   * e.g., "Expenses:Groceries:Organic" becomes nested structure
+   */
+  async getAccountHierarchy(rootAccount?: string): Promise<AccountNode[]> {
+    const accounts = await this.getAccounts()
+
+    // Filter by root account if specified
+    let filteredAccounts = accounts
+    if (rootAccount) {
+      filteredAccounts = accounts.filter(
+        (acc) =>
+          acc.name === rootAccount || acc.name.startsWith(`${rootAccount}:`)
+      )
+    }
+
+    // Build hierarchy
+    const accountMap = new Map<string, AccountNode>()
+
+    // First pass: Create nodes for all accounts
+    for (const account of filteredAccounts) {
+      const parts = account.name.split(':')
+      const level = parts.length - 1
+      const parentName = parts.slice(0, -1).join(':')
+
+      accountMap.set(account.name, {
+        account: {
+          ...account,
+          level,
+          parent_account: parentName || undefined,
+          has_children: false
+        },
+        children: []
+      })
+    }
+
+    // Second pass: Build parent-child relationships
+    const rootNodes: AccountNode[] = []
+
+    for (const [_name, node] of accountMap.entries()) {
+      const parentName = node.account.parent_account
+
+      if (parentName && accountMap.has(parentName)) {
+        // Add to parent's children
+        const parent = accountMap.get(parentName)!
+        parent.children.push(node)
+        parent.account.has_children = true
+      } else {
+        // Root level node
+        rootNodes.push(node)
+      }
+    }
+
+    // Sort children by name at each level
+    const sortNodes = (nodes: AccountNode[]) => {
+      nodes.sort((a, b) => a.account.name.localeCompare(b.account.name))
+      nodes.forEach((node) => sortNodes(node.children))
+    }
+    sortNodes(rootNodes)
+
+    return rootNodes
+  }
+
+  /**
+   * Submit expense entry to castle
+   */
+  async submitExpense(request: ExpenseEntryRequest): Promise<ExpenseEntry> {
+    try {
+      const response = await fetch(`${this.baseUrl}/api/v1/entries/expense`, {
+        method: 'POST',
+        headers: this.getHeaders(),
+        body: JSON.stringify(request),
+        signal: AbortSignal.timeout(this.config?.apiConfig?.timeout || 30000)
+      })
+
+      if (!response.ok) {
+        const errorData = await response.json().catch(() => ({}))
+        const errorMessage =
+          errorData.detail || `Failed to submit expense: ${response.statusText}`
+        throw new Error(errorMessage)
+      }
+
+      const entry = await response.json()
+
+      if (this.toastService) {
+        this.toastService.success('Expense submitted', {
+          description: 'Your expense is pending admin approval'
+        })
+      }
+
+      return entry as ExpenseEntry
+    } catch (error) {
+      console.error('[ExpensesAPI] Error submitting expense:', error)
+
+      let errorMessage = 'Failed to submit expense'
+      if (error instanceof Error) {
+        errorMessage = error.message
+      }
+
+      if (this.toastService) {
+        this.toastService.error('Submission failed', {
+          description: errorMessage
+        })
+      }
+
+      throw error
+    }
+  }
+
+  /**
+   * Get user's expense entries
+   */
+  async getUserExpenses(): Promise<ExpenseEntry[]> {
+    try {
+      const response = await fetch(`${this.baseUrl}/api/v1/entries/user`, {
+        method: 'GET',
+        headers: this.getHeaders(),
+        signal: AbortSignal.timeout(this.config?.apiConfig?.timeout || 30000)
+      })
+
+      if (!response.ok) {
+        throw new Error(
+          `Failed to fetch user expenses: ${response.statusText}`
+        )
+      }
+
+      const entries = await response.json()
+      return entries as ExpenseEntry[]
+    } catch (error) {
+      console.error('[ExpensesAPI] Error fetching user expenses:', error)
+      if (this.toastService) {
+        this.toastService.error('Failed to load expenses', {
+          description: error instanceof Error ? error.message : 'Unknown error'
+        })
+      }
+      throw error
+    }
+  }
+
+  /**
+   * Get user's balance with castle
+   */
+  async getUserBalance(): Promise<{ balance: number; currency: string }> {
+    try {
+      const response = await fetch(`${this.baseUrl}/api/v1/balance`, {
+        method: 'GET',
+        headers: this.getHeaders(),
+        signal: AbortSignal.timeout(this.config?.apiConfig?.timeout || 30000)
+      })
+
+      if (!response.ok) {
+        throw new Error(`Failed to fetch balance: ${response.statusText}`)
+      }
+
+      return await response.json()
+    } catch (error) {
+      console.error('[ExpensesAPI] Error fetching balance:', error)
+      throw error
+    }
+  }
+}
diff --git a/src/modules/expenses/types/index.ts b/src/modules/expenses/types/index.ts
new file mode 100644
index 0000000..04a4c97
--- /dev/null
+++ b/src/modules/expenses/types/index.ts
@@ -0,0 +1,97 @@
+/**
+ * Types for the Expenses module
+ */
+
+/**
+ * Account types in the castle double-entry accounting system
+ */
+export enum AccountType {
+  ASSET = 'asset',
+  LIABILITY = 'liability',
+  EQUITY = 'equity',
+  REVENUE = 'revenue',
+  EXPENSE = 'expense'
+}
+
+/**
+ * Account with hierarchical structure
+ */
+export interface Account {
+  id: string
+  name: string // e.g., "Expenses:Groceries:Organic"
+  account_type: AccountType
+  description?: string
+  user_id?: string
+  // Hierarchical metadata (added by frontend)
+  parent_account?: string
+  level?: number
+  has_children?: boolean
+}
+
+/**
+ * Account with user-specific permission metadata
+ * (Will be available once castle API implements permissions)
+ */
+export interface AccountWithPermissions extends Account {
+  user_permissions?: PermissionType[]
+  inherited_from?: string
+}
+
+/**
+ * Permission types for account access control
+ */
+export enum PermissionType {
+  READ = 'read',
+  SUBMIT_EXPENSE = 'submit_expense',
+  MANAGE = 'manage'
+}
+
+/**
+ * Expense entry request payload
+ */
+export interface ExpenseEntryRequest {
+  description: string
+  amount: number // In sats
+  expense_account: string // Account name or ID
+  is_equity: boolean
+  user_wallet: string
+  reference?: string
+  currency?: string // e.g., "USD"
+  entry_date?: string // ISO datetime string
+}
+
+/**
+ * Expense entry response from castle API
+ */
+export interface ExpenseEntry {
+  id: string
+  journal_id: string
+  description: string
+  amount: number
+  expense_account: string
+  is_equity: boolean
+  user_wallet: string
+  reference?: string
+  currency?: string
+  entry_date: string
+  created_at: string
+  status: 'pending' | 'approved' | 'rejected' | 'void'
+}
+
+/**
+ * Hierarchical account tree node for UI rendering
+ */
+export interface AccountNode {
+  account: Account
+  children: AccountNode[]
+}
+
+/**
+ * Module configuration
+ */
+export interface ExpensesConfig {
+  apiConfig: {
+    baseUrl: string
+    timeout: number
+  }
+}