diff --git a/src/main/kotlin/ltd/lulz/service/TransactionService.kt b/src/main/kotlin/ltd/lulz/service/TransactionService.kt
new file mode 100644
index 0000000..ca56cbd
--- /dev/null
+++ b/src/main/kotlin/ltd/lulz/service/TransactionService.kt
@@ -0,0 +1,26 @@
+package ltd.lulz.service
+
+import io.github.oshai.kotlinlogging.KotlinLogging
+import java.math.BigDecimal
+import java.util.UUID
+import ltd.lulz.model.AccountEntity
+import org.springframework.stereotype.Service
+import org.springframework.transaction.annotation.Transactional
+import reactor.core.publisher.Mono
+
+private val log = KotlinLogging.logger {}
+
+@Service
+class TransactionService(
+    private val accountService: AccountService,
+) {
+
+    @Transactional
+    fun deposit(
+        id: UUID,
+        amount: BigDecimal,
+    ): Mono<AccountEntity> = accountService.getForUpdateById(id)
+        .map { it.copy(amount = it.amount + amount) }
+        .doOnNext { log.trace { "Deposited $amount to account ${it.id}" } }
+        .flatMap { accountService.save(it) }
+}
diff --git a/src/test/kotlin/ltd/lulz/service/TransactionServiceTest.kt b/src/test/kotlin/ltd/lulz/service/TransactionServiceTest.kt
new file mode 100644
index 0000000..de42337
--- /dev/null
+++ b/src/test/kotlin/ltd/lulz/service/TransactionServiceTest.kt
@@ -0,0 +1,74 @@
+package ltd.lulz.service
+
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.slot
+import io.mockk.verify
+import java.math.BigDecimal
+import java.util.UUID
+import ltd.lulz.exception.AccountNotFoundException
+import ltd.lulz.model.AccountEntity
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+import reactor.core.publisher.Mono
+import reactor.test.StepVerifier
+
+@Suppress("MayBeConstant", "ReactiveStreamsUnusedPublisher")
+class TransactionServiceTest {
+
+    companion object {
+        val name: String = "some name"
+        val amount: BigDecimal = BigDecimal.valueOf(1.01)
+        val uuid: UUID = UUID.fromString("00000000-0000-0000-0000-000000000000")
+    }
+
+    private val accountService: AccountService = mockk()
+    private lateinit var service: TransactionService
+
+    @BeforeEach
+    fun setup() {
+        service = TransactionService(accountService)
+    }
+
+    @Test
+    fun `deposit to account - success`() {
+        // given
+        val deposit = BigDecimal.valueOf(1.10)
+
+        val capture = slot<UUID>()
+        every { accountService.getForUpdateById(capture(capture)) }
+            .answers { Mono.just(AccountEntity(capture.captured, name, amount)) }
+        val entity = slot<AccountEntity>()
+        every { accountService.save(capture(entity)) }
+            .answers { Mono.just(entity.captured) }
+
+        // when stepped
+        StepVerifier.create(service.deposit(uuid, deposit))
+            .assertNext { result ->
+                assertThat(result.id).isEqualTo(uuid)
+                assertThat(result.name).isEqualTo(name)
+                assertThat(result.amount).isEqualTo(amount + deposit)
+            }
+            .verifyComplete()
+
+        verify(exactly = 1) { accountService.getForUpdateById(any(UUID::class)) }
+        verify(exactly = 1) { accountService.save(any()) }
+    }
+
+    @Test
+    fun `deposit to account - account not found`() {
+        // given
+        val deposit = BigDecimal.valueOf(1.10)
+
+        every { accountService.getForUpdateById(any()) } returns Mono.error(AccountNotFoundException())
+
+        // when stepped
+        StepVerifier.create(service.deposit(uuid, deposit))
+            .expectError(AccountNotFoundException::class.java)
+            .verify()
+
+        verify(exactly = 1) { accountService.getForUpdateById(any(UUID::class)) }
+        verify(exactly = 0) { accountService.save(any()) }
+    }
+}