diff --git a/pom.xml b/pom.xml
index a8ffa1a..090a46e 100644
--- a/pom.xml
+++ b/pom.xml
@@ -95,6 +95,18 @@
             <artifactId>spring-boot-starter-test</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <version>4.13.1</version>
+            <scope>compile</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-api</artifactId>
+            <version>5.5.2</version>
+            <scope>compile</scope>
+        </dependency>
     </dependencies>
 
     <build>
diff --git a/src/main/java/com/devexperts/AccountServiceTest.java b/src/main/java/com/devexperts/AccountServiceTest.java
new file mode 100644
index 0000000..edd5ec5
--- /dev/null
+++ b/src/main/java/com/devexperts/AccountServiceTest.java
@@ -0,0 +1,59 @@
+package com.devexperts;
+
+import com.devexperts.account.Account;
+import com.devexperts.account.AccountKey;
+import com.devexperts.service.AccountService;
+import com.devexperts.service.AccountServiceImpl;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+
+public class AccountServiceTest {
+    private AccountService accountService;
+    private Account account;
+
+    @Before
+    public void setUp() {
+        accountService = new AccountServiceImpl();
+        account = new Account(AccountKey.valueOf(42), "FirstName", "LastName", 0.0);
+    }
+
+    @Test
+    public void testSingleAccountCreation() {
+        accountService.createAccount(account);
+        Account actual = accountService.getAccount(account.getAccountKey().getAccountId());
+        Assert.assertEquals(account, actual);
+    }
+
+    @Test
+    public void testAccountsCreation() {
+        accountService.createAccount(account);
+        Account actual = accountService.getAccount(account.getAccountKey().getAccountId());
+        Assert.assertEquals(account, actual);
+
+        Account otherAccount = new Account(AccountKey.valueOf(account.getAccountKey().getAccountId() + 1), "FirstName", "LastName", 0.0);
+        accountService.createAccount(otherAccount);
+        actual = accountService.getAccount(otherAccount.getAccountKey().getAccountId());
+        Assert.assertEquals(otherAccount, actual);
+        Assert.assertNotEquals(account, actual);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testExistingAccountCreation() {
+        Account account = new Account(AccountKey.valueOf(1), "FirstName", "LastName", 0.0);
+        accountService.createAccount(account);
+        accountService.createAccount(account);
+    }
+
+    @Test
+    public void testAccountsClearing() {
+        accountService.createAccount(account);
+        Account actual = accountService.getAccount(account.getAccountKey().getAccountId());
+        Assert.assertNotNull(actual);
+
+        accountService.clear();
+
+        actual = accountService.getAccount(account.getAccountKey().getAccountId());
+        Assert.assertNull(actual);
+    }
+}
diff --git a/src/main/java/com/devexperts/MultithreadTransferTest.java b/src/main/java/com/devexperts/MultithreadTransferTest.java
new file mode 100644
index 0000000..3badedc
--- /dev/null
+++ b/src/main/java/com/devexperts/MultithreadTransferTest.java
@@ -0,0 +1,43 @@
+package com.devexperts;
+
+import com.devexperts.account.Account;
+import com.devexperts.account.AccountKey;
+import com.devexperts.service.AccountServiceImpl;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class MultithreadTransferTest {
+    AccountServiceImpl service;
+    Account source, target;
+    AccountKey skey, tkey;
+
+    @BeforeEach
+    private void setUp() {
+        service = new AccountServiceImpl();
+        skey = AccountKey.valueOf(1);
+        source = new Account(skey, "One", "First", 100.0);
+
+        tkey = AccountKey.valueOf(2);
+        target = new Account(tkey, "Two", "Second", 200.0);
+    }
+
+    @Test
+    void multithreadTransfer() throws InterruptedException {
+        CountDownLatch latch = new CountDownLatch(10);
+        for (int i = 0; i < 10; i++) {
+            new Thread(()->{
+                service.transfer(source, target, 10.0);
+                latch.countDown();
+            }).start();
+        }
+        latch.await(1, TimeUnit.SECONDS);
+
+        assertEquals(0.0, source.getBalance());
+        assertEquals(300.0, target.getBalance());
+    }
+}
diff --git a/src/main/java/com/devexperts/TransferTest.java b/src/main/java/com/devexperts/TransferTest.java
new file mode 100644
index 0000000..b6429e1
--- /dev/null
+++ b/src/main/java/com/devexperts/TransferTest.java
@@ -0,0 +1,33 @@
+package com.devexperts;
+
+import com.devexperts.account.Account;
+import com.devexperts.account.AccountKey;
+import com.devexperts.service.AccountService;
+import com.devexperts.service.AccountServiceImpl;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class TransferTest {
+    private AccountService accountService;
+    private Account source;
+    private Account target;
+
+    @BeforeEach
+    public void setUp() {
+        accountService = new AccountServiceImpl();
+
+        source = new Account(AccountKey.valueOf(1), "One", "First", 100.0);
+        target = new Account(AccountKey.valueOf(2), "Two", "Second", 150.0);
+        accountService.createAccount(source);
+        accountService.createAccount(target);
+    }
+
+    @Test
+    public void transfer() {
+        accountService.transfer(source, target, 50.0);
+        assertEquals(source.getBalance(), 50.0);
+        assertEquals(target.getBalance(), 200.0);
+    }
+}
\ No newline at end of file
diff --git a/src/main/java/com/devexperts/account/AccountKey.java b/src/main/java/com/devexperts/account/AccountKey.java
index 1b0a233..1aae4af 100644
--- a/src/main/java/com/devexperts/account/AccountKey.java
+++ b/src/main/java/com/devexperts/account/AccountKey.java
@@ -1,5 +1,7 @@
 package com.devexperts.account;
 
+import java.util.Objects;
+
 /**
  * Unique Account identifier
  *
@@ -17,4 +19,21 @@ private AccountKey(long accountId) {
     public static AccountKey valueOf(long accountId) {
         return new AccountKey(accountId);
     }
+
+    public long getAccountId() {
+        return accountId;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        AccountKey that = (AccountKey) o;
+        return accountId == that.accountId;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(accountId);
+    }
 }
diff --git a/src/main/java/com/devexperts/rest/AbstractAccountController.java b/src/main/java/com/devexperts/rest/AbstractAccountController.java
index dea5a3c..f230808 100644
--- a/src/main/java/com/devexperts/rest/AbstractAccountController.java
+++ b/src/main/java/com/devexperts/rest/AbstractAccountController.java
@@ -3,5 +3,5 @@
 import org.springframework.http.ResponseEntity;
 
 public abstract class AbstractAccountController {
-    abstract ResponseEntity<Void> transfer(long sourceId, long targetId, double amount);
+    abstract ResponseEntity<Void> transfer(Long sourceId, Long targetId, Double amount);
 }
diff --git a/src/main/java/com/devexperts/rest/AccountController.java b/src/main/java/com/devexperts/rest/AccountController.java
index b300282..e13ebe2 100644
--- a/src/main/java/com/devexperts/rest/AccountController.java
+++ b/src/main/java/com/devexperts/rest/AccountController.java
@@ -1,14 +1,44 @@
 package com.devexperts.rest;
 
+import com.devexperts.account.Account;
+import com.devexperts.service.AccountService;
+import com.devexperts.service.exceptions.AccountNotFoundException;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RequestParam;
 import org.springframework.web.bind.annotation.RestController;
 
 @RestController
 @RequestMapping("/api")
 public class AccountController extends AbstractAccountController {
 
-    public ResponseEntity<Void> transfer(long sourceId, long targetId, double amount) {
-        return null;
+    private final AccountService service;
+
+    @Autowired
+    public AccountController (AccountService service) {
+        this.service = service;
+    }
+
+    @GetMapping("operations/transfer")
+    public ResponseEntity<Void> transfer(
+            @RequestParam("sourceId") Long sourceId,
+            @RequestParam("targetId") Long targetId,
+            @RequestParam("amount") Double amount) {
+        if (sourceId == null || targetId == null || amount < 0)
+            return new ResponseEntity<>(HttpStatus.BAD_REQUEST);
+
+        try {
+            Account source = service.getAccount(sourceId);
+            Account target = service.getAccount(targetId);
+            service.transfer(source, target, amount);
+            return new ResponseEntity<>(HttpStatus.OK);
+        } catch (AccountNotFoundException e) {
+            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
+        } catch (IllegalStateException e) {
+            return new ResponseEntity<>(HttpStatus.INTERNAL_SERVER_ERROR);
+        }
     }
-}
+}
\ No newline at end of file
diff --git a/src/main/java/com/devexperts/service/AccountServiceImpl.java b/src/main/java/com/devexperts/service/AccountServiceImpl.java
index 91261ba..1ced5a7 100644
--- a/src/main/java/com/devexperts/service/AccountServiceImpl.java
+++ b/src/main/java/com/devexperts/service/AccountServiceImpl.java
@@ -4,13 +4,15 @@
 import com.devexperts.account.AccountKey;
 import org.springframework.stereotype.Service;
 
-import java.util.ArrayList;
-import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.locks.ReentrantLock;
 
 @Service
 public class AccountServiceImpl implements AccountService {
 
-    private final List<Account> accounts = new ArrayList<>();
+    private final Map<AccountKey, Account> accounts = new ConcurrentHashMap<>();
+    private final ReentrantLock lock = new ReentrantLock();
 
     @Override
     public void clear() {
@@ -19,19 +21,38 @@ public void clear() {
 
     @Override
     public void createAccount(Account account) {
-        accounts.add(account);
+        if (account.getAccountKey() == null) {
+            throw new NullPointerException();
+        }
+        if (accounts.containsKey(account.getAccountKey())) {
+            throw new IllegalArgumentException("This key is already used.");
+        }
+        accounts.put(account.getAccountKey(), account);
     }
 
     @Override
     public Account getAccount(long id) {
-        return accounts.stream()
-                .filter(account -> account.getAccountKey() == AccountKey.valueOf(id))
-                .findAny()
-                .orElse(null);
+        return accounts.getOrDefault(AccountKey.valueOf(id), null);
     }
 
     @Override
     public void transfer(Account source, Account target, double amount) {
-        //do nothing for now
+        if (amount < 0 || source == target)
+            throw new IllegalArgumentException("Amount can't be negative OR Source can't be same as target.");
+
+        lock.lock();
+        try {
+            double sourceBalance = source.getBalance();
+            double targetBalance = target.getBalance();
+
+            if (sourceBalance < amount)
+                throw new IllegalStateException("Source balance is insufficient.");
+
+            source.setBalance(sourceBalance - amount);
+            target.setBalance(targetBalance + amount);
+
+        } finally {
+            lock.unlock();
+        }
     }
 }
diff --git a/src/main/java/com/devexperts/service/exceptions/AccountNotFoundException.java b/src/main/java/com/devexperts/service/exceptions/AccountNotFoundException.java
new file mode 100644
index 0000000..e0a217b
--- /dev/null
+++ b/src/main/java/com/devexperts/service/exceptions/AccountNotFoundException.java
@@ -0,0 +1,7 @@
+package com.devexperts.service.exceptions;
+
+public class AccountNotFoundException extends RuntimeException {
+    public AccountNotFoundException(String message) {
+        super(message);
+    }
+}
diff --git a/src/main/java/resources/data/accounts.sql b/src/main/java/resources/data/accounts.sql
new file mode 100644
index 0000000..29f4e9d
--- /dev/null
+++ b/src/main/java/resources/data/accounts.sql
@@ -0,0 +1,7 @@
+CREATE TABLE IF NOT EXISTS Accounts
+(
+    ID BIGINT AUTO_INCREMENT PRIMARY,
+    FIRST_NAME VARCHAR(255) NOT NULL,
+    LAST_NAME VARCHAR(255) NOT NULL,
+    BALANCE BIGINT NOT NULL
+    );
\ No newline at end of file
diff --git a/src/main/java/resources/data/select.sql b/src/main/java/resources/data/select.sql
new file mode 100644
index 0000000..7c39e59
--- /dev/null
+++ b/src/main/java/resources/data/select.sql
@@ -0,0 +1,4 @@
+SELECT t.source_id FROM Transfers AS t
+WHERE t.transfer_time >= TIMESTAMP '2019-01-01' AND source_id != target_id AND amount > 0
+GROUP BY source_id
+HAVING SUM(amount) > 1000;
\ No newline at end of file
diff --git a/src/main/java/resources/transfers.sql b/src/main/java/resources/transfers.sql
new file mode 100644
index 0000000..1d477a4
--- /dev/null
+++ b/src/main/java/resources/transfers.sql
@@ -0,0 +1,10 @@
+CREATE TABLE IF NOT EXISTS Transfers
+(
+    ID BIGINT AUTO_INCREMENT PRIMARY,
+    SOURCE_ID BIGINT NOT NULL,
+    TARGET_ID BIGINT NOT NULL,
+    AMOUNT BIGINT NOT NULL,
+    TRANSFER_TIME TIMESTAMP NOT NULL,
+    FOREIGN KEY (SOURCE_ID) REFERENCES accounts (ID),
+    FOREIGN KEY (TARGET_ID) REFERENCES accounts (ID)
+    );
\ No newline at end of file