diff --git a/src/test/java/com/example/bankapp/service/AccountServiceTest.java b/src/test/java/com/example/bankapp/service/AccountServiceTest.java
new file mode 100644
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+++ b/src/test/java/com/example/bankapp/service/AccountServiceTest.java
@@ -0,0 +1,259 @@
+package com.example.bankapp.service;
+
+import com.example.bankapp.model.Account;
+import com.example.bankapp.model.Transaction;
+import com.example.bankapp.repository.AccountRepository;
+import com.example.bankapp.repository.TransactionRepository;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.security.core.userdetails.UserDetails;
+import org.springframework.security.crypto.password.PasswordEncoder;
+
+import java.math.BigDecimal;
+import java.time.LocalDateTime;
+import java.util.List;
+import java.util.Optional;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+class AccountServiceTest {
+
+    @Mock
+    private AccountRepository accountRepository;
+
+    @Mock
+    private TransactionRepository transactionRepository;
+
+    @Mock
+    private PasswordEncoder passwordEncoder;
+
+    @InjectMocks
+    private AccountService accountService;
+
+    private Account account;
+
+    @BeforeEach
+    void setUp() {
+        account = new Account();
+        account.setId(1L);
+        account.setUsername("alice");
+        account.setPassword("secret");
+        account.setBalance(new BigDecimal("100"));
+    }
+
+    // ----- deposit() -----
+
+    @Test
+    void deposit_shouldIncreaseBalanceByAmount() {
+        accountService.deposit(account, new BigDecimal("50"));
+
+        assertEquals(new BigDecimal("150"), account.getBalance());
+        verify(accountRepository).save(account);
+        verify(transactionRepository).save(any(Transaction.class));
+    }
+
+    @Test
+    void deposit_shouldCreateTransactionWithCorrectType() {
+        accountService.deposit(account, new BigDecimal("50"));
+
+        ArgumentCaptor<Transaction> captor = ArgumentCaptor.forClass(Transaction.class);
+        verify(transactionRepository).save(captor.capture());
+
+        Transaction saved = captor.getValue();
+        assertEquals("Deposit", saved.getType());
+        assertEquals(new BigDecimal("50"), saved.getAmount());
+    }
+
+    // ----- withdraw() -----
+
+    @Test
+    void withdraw_shouldDecreaseBalanceByAmount() {
+        accountService.withdraw(account, new BigDecimal("30"));
+
+        assertEquals(new BigDecimal("70"), account.getBalance());
+        verify(accountRepository).save(account);
+        verify(transactionRepository).save(any(Transaction.class));
+    }
+
+    @Test
+    void withdraw_shouldThrowRuntimeExceptionWhenInsufficientFunds() {
+        account.setBalance(new BigDecimal("10"));
+
+        RuntimeException ex = assertThrows(RuntimeException.class,
+                () -> accountService.withdraw(account, new BigDecimal("50")));
+
+        assertEquals("Insufficient funds", ex.getMessage());
+        verify(accountRepository, never()).save(any(Account.class));
+        verify(transactionRepository, never()).save(any(Transaction.class));
+    }
+
+    @Test
+    void withdraw_shouldCreateTransactionWithCorrectType() {
+        accountService.withdraw(account, new BigDecimal("30"));
+
+        ArgumentCaptor<Transaction> captor = ArgumentCaptor.forClass(Transaction.class);
+        verify(transactionRepository).save(captor.capture());
+
+        Transaction saved = captor.getValue();
+        assertEquals("Withdrawal", saved.getType());
+        assertEquals(new BigDecimal("30"), saved.getAmount());
+    }
+
+    // ----- transferAmount() -----
+
+    @Test
+    void transferAmount_shouldDeductFromSenderAndAddToRecipient() {
+        account.setBalance(new BigDecimal("200"));
+
+        Account recipient = new Account();
+        recipient.setId(2L);
+        recipient.setUsername("bob");
+        recipient.setBalance(new BigDecimal("0"));
+
+        when(accountRepository.findByUsername("bob")).thenReturn(Optional.of(recipient));
+
+        accountService.transferAmount(account, "bob", new BigDecimal("50"));
+
+        assertEquals(new BigDecimal("150"), account.getBalance());
+        assertEquals(new BigDecimal("50"), recipient.getBalance());
+    }
+
+    @Test
+    void transferAmount_shouldThrowWhenInsufficientFunds() {
+        account.setBalance(new BigDecimal("10"));
+
+        RuntimeException ex = assertThrows(RuntimeException.class,
+                () -> accountService.transferAmount(account, "bob", new BigDecimal("50")));
+
+        assertEquals("Insufficient funds", ex.getMessage());
+        verify(transactionRepository, never()).save(any(Transaction.class));
+    }
+
+    @Test
+    void transferAmount_shouldThrowWhenRecipientNotFound() {
+        account.setBalance(new BigDecimal("200"));
+
+        when(accountRepository.findByUsername("bob")).thenReturn(Optional.empty());
+
+        RuntimeException ex = assertThrows(RuntimeException.class,
+                () -> accountService.transferAmount(account, "bob", new BigDecimal("50")));
+
+        assertEquals("Recipient account not found", ex.getMessage());
+    }
+
+    @Test
+    void transferAmount_shouldCreateTwoTransactionRecords() {
+        account.setBalance(new BigDecimal("200"));
+
+        Account recipient = new Account();
+        recipient.setId(2L);
+        recipient.setUsername("bob");
+        recipient.setBalance(new BigDecimal("0"));
+
+        when(accountRepository.findByUsername("bob")).thenReturn(Optional.of(recipient));
+
+        accountService.transferAmount(account, "bob", new BigDecimal("50"));
+
+        verify(transactionRepository, times(2)).save(any(Transaction.class));
+    }
+
+    // ----- registerAccount() -----
+
+    @Test
+    void registerAccount_shouldCreateAccountWithZeroBalance() {
+        when(accountRepository.findByUsername("newuser")).thenReturn(Optional.empty());
+        when(passwordEncoder.encode("password")).thenReturn("encoded");
+        when(accountRepository.save(any(Account.class))).thenAnswer(invocation -> invocation.getArgument(0));
+
+        accountService.registerAccount("newuser", "password");
+
+        ArgumentCaptor<Account> captor = ArgumentCaptor.forClass(Account.class);
+        verify(accountRepository).save(captor.capture());
+
+        Account saved = captor.getValue();
+        assertEquals("newuser", saved.getUsername());
+        assertEquals("encoded", saved.getPassword());
+        assertEquals(BigDecimal.ZERO, saved.getBalance());
+    }
+
+    @Test
+    void registerAccount_shouldThrowWhenUsernameAlreadyExists() {
+        when(accountRepository.findByUsername("alice")).thenReturn(Optional.of(account));
+
+        RuntimeException ex = assertThrows(RuntimeException.class,
+                () -> accountService.registerAccount("alice", "password"));
+
+        assertEquals("Username already exists", ex.getMessage());
+        verify(accountRepository, never()).save(any(Account.class));
+    }
+
+    // ----- findAccountByUsername() -----
+
+    @Test
+    void findAccountByUsername_shouldReturnAccountWhenFound() {
+        when(accountRepository.findByUsername("alice")).thenReturn(Optional.of(account));
+
+        Account result = accountService.findAccountByUsername("alice");
+
+        assertSame(account, result);
+    }
+
+    @Test
+    void findAccountByUsername_shouldThrowWhenNotFound() {
+        when(accountRepository.findByUsername("ghost")).thenReturn(Optional.empty());
+
+        RuntimeException ex = assertThrows(RuntimeException.class,
+                () -> accountService.findAccountByUsername("ghost"));
+
+        assertEquals("Account not found", ex.getMessage());
+    }
+
+    // ----- loadUserByUsername() -----
+
+    @Test
+    void loadUserByUsername_shouldReturnUserDetailsWhenFound() {
+        when(accountRepository.findByUsername("alice")).thenReturn(Optional.of(account));
+
+        UserDetails userDetails = accountService.loadUserByUsername("alice");
+
+        assertEquals("alice", userDetails.getUsername());
+        assertEquals("secret", userDetails.getPassword());
+    }
+
+    @Test
+    void loadUserByUsername_shouldThrowWhenUsernameNotFound() {
+        when(accountRepository.findByUsername("ghost")).thenReturn(Optional.empty());
+
+        assertThrows(RuntimeException.class,
+                () -> accountService.loadUserByUsername("ghost"));
+    }
+
+    // ----- getTransactionHistory() -----
+
+    @Test
+    void getTransactionHistory_shouldReturnTransactionsForAccount() {
+        Transaction t1 = new Transaction(new BigDecimal("50"), "Deposit", LocalDateTime.now(), account);
+        Transaction t2 = new Transaction(new BigDecimal("30"), "Withdrawal", LocalDateTime.now(), account);
+        List<Transaction> transactions = List.of(t1, t2);
+
+        when(transactionRepository.findByAccountId(1L)).thenReturn(transactions);
+
+        List<Transaction> result = accountService.getTransactionHistory(account);
+
+        assertEquals(2, result.size());
+        assertEquals(transactions, result);
+    }
+}