diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index 48d598a..06d8a68 100644
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -27,14 +27,23 @@ MoneyMap follows a clean, service-oriented architecture:
                  │
 ┌────────────────┴────────────────────────────┐
 │         Service Layer (Business Logic)      │
-│  - TransactionImporter                      │
-│  - CardResolver                             │
-│  - TransactionCategorizer                   │
-│  - TransactionService (NEW)                 │
-│  - ReceiptMatchingService (NEW)             │
-│  - DashboardService                         │
-│  - ReceiptManager                           │
-│  - OpenAIReceiptParser                      │
+│  Core Transaction Services:                 │
+│    - TransactionImporter                    │
+│    - CardResolver                           │
+│    - TransactionCategorizer                 │
+│    - TransactionService (NEW)               │
+│    - TransactionStatisticsService (NEW)     │
+│  Entity Management Services:                │
+│    - AccountService (NEW)                   │
+│    - CardService (NEW)                      │
+│    - MerchantService (EXPANDED)             │
+│  Receipt Services:                          │
+│    - ReceiptMatchingService (NEW)           │
+│    - ReceiptManager                         │
+│    - AIReceiptParser                        │
+│  Reference & Dashboard:                     │
+│    - ReferenceDataService (NEW)             │
+│    - DashboardService                       │
 └────────────────┬────────────────────────────┘
                  │
 ┌────────────────┴────────────────────────────┐
@@ -330,6 +339,180 @@ Pattern-based rules for auto-categorization with merchant linking.
 
 **Location:** Services/ReceiptMatchingService.cs
 
+### AccountService (Services/AccountService.cs)
+**Interface:** `IAccountService`
+
+**Responsibility:** Account management including retrieval, validation, and deletion.
+
+**Key Methods:**
+- `GetAccountByIdAsync(int id, bool includeRelated = false)`
+  - Retrieves account by ID with optional related data (cards, transactions)
+  - Returns Account entity or null
+
+- `GetAllAccountsWithStatsAsync()`
+  - Gets all accounts with transaction counts
+  - Returns list of `AccountWithStats` DTOs
+  - Sorted by owner, institution, last4
+
+- `GetAccountDetailsAsync(int id)`
+  - Gets comprehensive account details including linked cards and stats
+  - Returns `AccountDetails` DTO with cards and transaction counts
+
+- `CanDeleteAccountAsync(int id)`
+  - Validates if account can be deleted (no transactions)
+  - Returns `DeleteValidationResult`
+
+- `DeleteAccountAsync(int id)`
+  - Deletes account after validation
+  - Returns `DeleteResult` with success/failure message
+
+**DTOs:**
+- `AccountWithStats` - Account with transaction count
+- `AccountDetails` - Full account details with cards and statistics
+- `DeleteValidationResult` - Validation result with reason if cannot delete
+- `DeleteResult` - Operation result with success status and message
+
+**Design Pattern:**
+- Consolidates account deletion validation logic from multiple PageModels
+- Provides single source of truth for account operations
+- Improves testability
+
+**Location:** Services/AccountService.cs
+
+### CardService (Services/CardService.cs)
+**Interface:** `ICardService`
+
+**Responsibility:** Card management including retrieval, validation, and deletion.
+
+**Key Methods:**
+- `GetCardByIdAsync(int id, bool includeRelated = false)`
+  - Retrieves card by ID with optional related data (account, transactions)
+  - Returns Card entity or null
+
+- `GetAllCardsWithStatsAsync()`
+  - Gets all cards with transaction counts
+  - Returns list of `CardWithStats` DTOs
+  - Sorted by owner, last4
+
+- `CanDeleteCardAsync(int id)`
+  - Validates if card can be deleted (no transactions)
+  - Returns `DeleteValidationResult`
+
+- `DeleteCardAsync(int id)`
+  - Deletes card after validation
+  - Returns `DeleteResult` with success/failure message
+
+**DTOs:**
+- `CardWithStats` - Card with transaction count
+
+**Design Pattern:**
+- Consolidates card deletion validation logic from Cards and AccountDetails pages
+- Mirrors AccountService for consistency
+- Enables reusable card management operations
+
+**Location:** Services/CardService.cs
+
+### MerchantService (Services/MerchantService.cs) - EXPANDED
+**Interface:** `IMerchantService`
+
+**Responsibility:** Merchant management including creation, retrieval, update, and deletion.
+
+**Existing Methods:**
+- `FindByNameAsync(string name)` - Find merchant by exact name match
+- `GetOrCreateAsync(string name)` - Get existing or create new merchant
+- `GetOrCreateIdAsync(string? name)` - Get or create merchant and return ID
+
+**New Methods:**
+- `GetMerchantByIdAsync(int id, bool includeRelated = false)`
+  - Retrieves merchant by ID with optional related data (transactions, category mappings)
+
+- `GetAllMerchantsWithStatsAsync()`
+  - Gets all merchants with transaction and mapping counts
+  - Returns list of `MerchantWithStats` DTOs
+  - Sorted alphabetically by name
+
+- `UpdateMerchantAsync(int id, string newName)`
+  - Updates merchant name with duplicate checking
+  - Returns `MerchantUpdateResult`
+
+- `DeleteMerchantAsync(int id)`
+  - Deletes merchant and reports unlinked transactions/mappings
+  - Returns `MerchantDeleteResult` with counts
+
+**DTOs:**
+- `MerchantWithStats` - Merchant with transaction and mapping counts
+- `MerchantUpdateResult` - Update operation result
+- `MerchantDeleteResult` - Delete operation result with impact statistics
+
+**Design Pattern:**
+- Expands existing service with full CRUD operations
+- Used by EditTransaction for merchant creation
+- Used by Merchants page for management UI
+
+**Location:** Services/MerchantService.cs
+
+### TransactionStatisticsService (Services/TransactionStatisticsService.cs)
+**Interface:** `ITransactionStatisticsService`
+
+**Responsibility:** Calculate transaction statistics and aggregates.
+
+**Key Methods:**
+- `CalculateStatsAsync(IQueryable<Transaction> query)`
+  - Calculates statistics for any filtered set of transactions
+  - Returns `TransactionStats` DTO with counts and totals
+  - Computes: total count, total debits, total credits, net amount
+
+- `GetCategorizationStatsAsync()`
+  - Gets categorization statistics for entire database
+  - Returns `CategorizationStats` with categorized/uncategorized counts
+
+- `GetCardStatsForAccountAsync(int accountId)`
+  - Gets transaction counts for all cards linked to an account
+  - Returns list of `CardStats` DTOs
+
+**DTOs:**
+- `TransactionStats` - Aggregate statistics (count, debits, credits, net)
+- `CategorizationStats` - Categorization summary
+- `CardStats` - Card with transaction count
+
+**Design Pattern:**
+- Consolidates statistics calculation from Transactions, Recategorize, and AccountDetails pages
+- Enables consistent statistics across the application
+- Improves performance by centralizing query logic
+
+**Location:** Services/TransactionStatisticsService.cs
+
+### ReferenceDataService (Services/ReferenceDataService.cs)
+**Interface:** `IReferenceDataService`
+
+**Responsibility:** Provide reference/lookup data for dropdowns and filters.
+
+**Key Methods:**
+- `GetAvailableCategoriesAsync()`
+  - Gets all distinct categories from transactions
+  - Returns sorted list of category names
+  - Excludes empty/whitespace categories
+
+- `GetAvailableMerchantsAsync()`
+  - Gets all merchants sorted by name
+  - Returns list of Merchant entities
+
+- `GetAvailableCardsAsync(bool includeAccount = true)`
+  - Gets all cards with optional account information
+  - Returns sorted list of Card entities
+  - Sorted by owner, last4
+
+- `GetAvailableAccountsAsync()`
+  - Gets all accounts sorted by institution and last4
+  - Returns list of Account entities
+
+**Design Pattern:**
+- Centralizes repetitive reference data queries used across multiple pages
+- Reduces code duplication in Transactions, EditTransaction, Upload pages
+- Provides consistent sorting and filtering
+
+**Location:** Services/ReferenceDataService.cs
+
 ### DashboardService (Pages/Index.cshtml.cs)
 **Interface:** `IDashboardService`