diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..8acb57b
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,5 @@
+# build output directory
+target/
+
+# IntelliJ generated files
+.idea/
diff --git a/pom.xml b/pom.xml
index 2ca15ec..5344dd2 100644
--- a/pom.xml
+++ b/pom.xml
@@ -3,10 +3,47 @@
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
     <modelVersion>4.0.0</modelVersion>
-
     <groupId>ch.engenius</groupId>
     <artifactId>accounts</artifactId>
     <version>1.0-SNAPSHOT</version>
-
-
+    <properties>
+        <maven.compiler.source>11</maven.compiler.source>
+        <maven.compiler.target>11</maven.compiler.target>
+    </properties>
+    <build>
+        <plugins>
+            <plugin>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>2.19.1</version>
+                <dependencies>
+                    <dependency>
+                        <groupId>org.junit.platform</groupId>
+                        <artifactId>junit-platform-surefire-provider</artifactId>
+                        <version>1.0.3</version>
+                    </dependency>
+                    <dependency>
+                        <groupId>org.junit.jupiter</groupId>
+                        <artifactId>junit-jupiter-engine</artifactId>
+                        <version>5.8.1</version>
+                    </dependency>
+                </dependencies>
+            </plugin>
+            <plugin>
+                <groupId>net.revelc.code.formatter</groupId>
+                <artifactId>formatter-maven-plugin</artifactId>
+                <version>2.18.0</version>
+                <configuration>
+                    <encoding>UTF-8</encoding>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-api</artifactId>
+            <version>5.8.1</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
 </project>
\ No newline at end of file
diff --git a/src/main/java/ch/engenius/bank/Account.java b/src/main/java/ch/engenius/bank/Account.java
index b9979cb..74549f6 100644
--- a/src/main/java/ch/engenius/bank/Account.java
+++ b/src/main/java/ch/engenius/bank/Account.java
@@ -3,29 +3,69 @@
 import java.math.BigDecimal;
 
 public class Account {
-    private double money;
+    private BigDecimal money;
 
-    public void withdraw(double amount) {
-        if ((money - amount) < 0) {
-            throw new IllegalStateException("not enough credits on account");
+    /*
+     * Creates the account with zero credit.
+     */
+    public Account() {
+        this(BigDecimal.ZERO);
+    }
+
+    /*
+     * Creates the account with given amount of credit.
+     *
+     * @param money Initial amount of credit for the account
+     */
+    public Account(BigDecimal money) {
+        this.money = money;
+    }
+
+    /*
+     * Withdraws given amount of credit from the account.
+     *
+     * @param amount Amount to withdraw
+     *
+     * @throws IllegalArgumentException If given amount is bellow or equal zero
+     *
+     * @throws IllegalStateException If given amount is larger than credits on the account
+     */
+    public void withdraw(BigDecimal amount) {
+        if (amount.compareTo(BigDecimal.ZERO) < 1) {
+            throw new IllegalArgumentException("cannot withdraw zero or negative amount");
         }
-        setMoney(money - amount);
 
+        synchronized (this) {
+            if (money.subtract(amount).compareTo(BigDecimal.ZERO) == -1) {
+                throw new IllegalStateException("not enough credit");
+            }
+
+            setMoney(money.subtract(amount));
+        }
     }
 
-    public void deposit(double amount) {
-        setMoney(money + amount);
+    /*
+     * Deposits given amount of credit to the account.
+     *
+     * @param amount Amount to deposit
+     *
+     * @throws IllegalArgumentException If given amount is bellow or equal zero
+     */
+    public void deposit(BigDecimal amount) {
+        if (amount.compareTo(BigDecimal.ZERO) < 1) {
+            throw new IllegalArgumentException("cannot deposit zero or negative amount");
+        }
+
+        synchronized (this) {
+            setMoney(money.add(amount));
+        }
     }
 
-    public double getMoney() {
+    public BigDecimal getMoney() {
         return money;
     }
 
-    public void setMoney(double money) {
+    private void setMoney(BigDecimal money) {
         this.money = money;
     }
-
-    public BigDecimal getMoneyAsBigDecimal() {
-        return BigDecimal.valueOf(money);
-    }
 }
diff --git a/src/main/java/ch/engenius/bank/Bank.java b/src/main/java/ch/engenius/bank/Bank.java
index 571ebc7..2eccf04 100644
--- a/src/main/java/ch/engenius/bank/Bank.java
+++ b/src/main/java/ch/engenius/bank/Bank.java
@@ -1,18 +1,42 @@
 package ch.engenius.bank;
 
+import java.math.BigDecimal;
 import java.util.HashMap;
 
 public class Bank {
     private HashMap<Integer, Account> accounts = new HashMap<>();
 
-    public Account registerAccount(int accountNumber, int amount) {
-        Account account = new Account();
-        account.setMoney(amount);
+    /*
+     * Creates an account with specified account number and initial amount of credits.
+     *
+     * @param accountNumber Number of the account
+     *
+     * @param amount Initial credit for the account
+     *
+     * @thorws IllegalStateException If account with given number has already been registered
+     */
+    public Account registerAccount(int accountNumber, BigDecimal amount) {
+        if (accounts.containsKey(accountNumber)) {
+            throw new IllegalStateException("account already exists");
+        }
+
+        Account account = new Account(amount);
         accounts.put(accountNumber, account);
         return account;
     }
 
-    public Account getAccount( int number) {
-        return accounts.get(number);
+    /*
+     * Retrieve account by its number
+     *
+     * @param accountNumber Number of the account to retrieve
+     *
+     * @throws IllegalStateException If account with given number is not registered
+     */
+    public Account getAccount(int accountNumber) {
+        if (!accounts.containsKey(accountNumber)) {
+            throw new IllegalStateException("account does not exist");
+        }
+
+        return accounts.get(accountNumber);
     }
 }
diff --git a/src/main/java/ch/engenius/bank/BankRunner.java b/src/main/java/ch/engenius/bank/BankRunner.java
index 10b30fd..093e857 100644
--- a/src/main/java/ch/engenius/bank/BankRunner.java
+++ b/src/main/java/ch/engenius/bank/BankRunner.java
@@ -1,6 +1,7 @@
 package ch.engenius.bank;
 
 import java.math.BigDecimal;
+import java.util.List;
 import java.util.Random;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -14,58 +15,57 @@ public class BankRunner {
     private final Random random = new Random(43);
     private final Bank bank = new Bank();
 
-
     public static void main(String[] args) {
         BankRunner runner = new BankRunner();
         int accounts = 100;
-        int defaultDeposit  = 1000;
-        int iterations  = 10000;
+        int defaultDeposit = 1000;
+        int iterations = 10000;
         runner.registerAccounts(accounts, defaultDeposit);
-        runner.sanityCheck(accounts, accounts*defaultDeposit);
+        runner.sanityCheck(accounts, accounts * defaultDeposit);
         runner.runBank(iterations, accounts);
-        runner.sanityCheck(accounts, accounts*defaultDeposit);
-
+        runner.sanityCheck(accounts, accounts * defaultDeposit);
     }
 
     private void runBank(int iterations, int maxAccount) {
-        for (int i =0; i< iterations; i++ ) {
-            executor.submit( ()-> runRandomOperation(maxAccount));
+        for (int i = 0; i < iterations; i++) {
+            executor.submit(() -> runRandomOperation(maxAccount));
         }
         try {
             executor.shutdown();
-            executor.awaitTermination(100,TimeUnit.SECONDS);
+            executor.awaitTermination(1, TimeUnit.SECONDS);
         } catch (InterruptedException e) {
             e.printStackTrace();
+        } finally {
+            if (!executor.isTerminated()) {
+                List<Runnable> operations = executor.shutdownNow();
+                System.err.println("# of operations that failed to complete: " + operations.size());
+            }
         }
     }
 
     private void runRandomOperation(int maxAccount) {
-        double transfer = random.nextDouble()*100.0;
+        BigDecimal transfer = BigDecimal.valueOf(random.nextDouble()).multiply(BigDecimal.valueOf(100));
         int accountInNumber = random.nextInt(maxAccount);
         int accountOutNumber = random.nextInt(maxAccount);
-        Account accIn  =bank.getAccount(accountInNumber);
-        Account accOut  =bank.getAccount(accountOutNumber);
-        accIn.deposit(transfer);
+        Account accIn = bank.getAccount(accountInNumber);
+        Account accOut = bank.getAccount(accountOutNumber);
         accOut.withdraw(transfer);
+        accIn.deposit(transfer);
     }
 
-    private void  registerAccounts(int number, int defaultMoney) {
-        for ( int i = 0; i < number; i++) {
-            bank.registerAccount(i, defaultMoney);
+    private void registerAccounts(int number, int defaultMoney) {
+        for (int i = 0; i < number; i++) {
+            bank.registerAccount(i, BigDecimal.valueOf(defaultMoney));
         }
     }
 
-    private void sanityCheck( int accountMaxNumber, int totalExpectedMoney) {
-        BigDecimal sum = IntStream.range(0, accountMaxNumber)
-                .mapToObj( bank::getAccount)
-                .map ( Account::getMoneyAsBigDecimal)
-                .reduce( BigDecimal.ZERO, BigDecimal::add);
+    private void sanityCheck(int accountMaxNumber, int totalExpectedMoney) {
+        BigDecimal sum = IntStream.range(0, accountMaxNumber).mapToObj(bank::getAccount).map(Account::getMoney)
+                .reduce(BigDecimal.ZERO, BigDecimal::add);
 
-        if ( sum.intValue() != totalExpectedMoney) {
-            throw new IllegalStateException("we got "+ sum + " != " + totalExpectedMoney +" (expected)");
+        if (sum.intValue() != totalExpectedMoney) {
+            throw new IllegalStateException("we got " + sum + " != " + totalExpectedMoney + " (expected)");
         }
         System.out.println("sanity check OK");
     }
-
-
 }
diff --git a/src/test/java/ch/engenius/bank/AccountTest.java b/src/test/java/ch/engenius/bank/AccountTest.java
new file mode 100644
index 0000000..86d0c80
--- /dev/null
+++ b/src/test/java/ch/engenius/bank/AccountTest.java
@@ -0,0 +1,80 @@
+package ch.engenius.bank;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.BeforeEach;
+import java.math.BigDecimal;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class AccountTest {
+
+    private Account account;
+    private final BigDecimal initialMoney = BigDecimal.valueOf(123.456);
+
+    @BeforeEach
+    public void setUp() {
+        account = new Account(initialMoney);
+    }
+
+    @Test
+    public void shouldDepositMoney() {
+        final BigDecimal depositAmount = BigDecimal.valueOf(11);
+
+        account.deposit(depositAmount);
+
+        assertEquals(initialMoney.add(depositAmount), account.getMoney());
+    }
+
+    @Test
+    public void shouldWithdrawMoney() {
+        final BigDecimal withdrawalAmount = BigDecimal.valueOf(22);
+
+        account.withdraw(withdrawalAmount);
+
+        assertEquals(initialMoney.subtract(withdrawalAmount), account.getMoney());
+    }
+
+    @Test
+    public void shouldFailToDepositNothing() {
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
+                () -> account.deposit(BigDecimal.ZERO));
+        assertEquals("cannot deposit zero or negative amount", exception.getMessage());
+    }
+
+    @Test
+    public void shouldFailToDepositNegativeAmount() {
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
+                () -> account.deposit(BigDecimal.valueOf(-100)));
+        assertEquals("cannot deposit zero or negative amount", exception.getMessage());
+    }
+
+    @Test
+    public void shouldWithdrawAllMoney() {
+        account.withdraw(initialMoney);
+
+        assertEquals(BigDecimal.ZERO, account.getMoney().setScale(0));
+    }
+
+    @Test
+    public void shouldFailToWithdrawNothing() {
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
+                () -> account.withdraw(BigDecimal.ZERO));
+        assertEquals("cannot withdraw zero or negative amount", exception.getMessage());
+    }
+
+    @Test
+    public void shouldFailToWithdrawNegativeAmount() {
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
+                () -> account.withdraw(BigDecimal.valueOf(-100)));
+        assertEquals("cannot withdraw zero or negative amount", exception.getMessage());
+    }
+
+    @Test
+    public void shouldFailToWithdrawWhenMissingMoney() {
+        final BigDecimal withdrawalAmount = BigDecimal.valueOf(321);
+
+        IllegalStateException exception = assertThrows(IllegalStateException.class,
+                () -> account.withdraw(BigDecimal.valueOf(321)));
+        assertEquals("not enough credit", exception.getMessage());
+    }
+}
diff --git a/src/test/java/ch/engenius/bank/BankTest.java b/src/test/java/ch/engenius/bank/BankTest.java
new file mode 100644
index 0000000..d2d8c7d
--- /dev/null
+++ b/src/test/java/ch/engenius/bank/BankTest.java
@@ -0,0 +1,48 @@
+package ch.engenius.bank;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Order;
+import org.junit.jupiter.api.Test;
+import java.math.BigDecimal;
+import static org.junit.jupiter.api.Assertions.*;
+
+public class BankTest {
+
+    private static Bank bank;
+    private final int accountNumber = 3;
+    private final BigDecimal amount = BigDecimal.valueOf(123.456);
+
+    @BeforeAll
+    public static void setUp() {
+        bank = new Bank();
+    }
+
+    @Test
+    @Order(1)
+    public void shouldRegisterAccount() {
+        bank.registerAccount(accountNumber, amount);
+
+        Account account = bank.getAccount(accountNumber);
+        assertNotNull(account);
+    }
+
+    @Test
+    public void shouldFailToRegisterAccountIfAlreadyExists() {
+        IllegalStateException exception = assertThrows(IllegalStateException.class,
+                () -> bank.registerAccount(accountNumber, BigDecimal.ZERO));
+        assertEquals("account already exists", exception.getMessage());
+    }
+
+    @Test
+    public void shouldRegisterAccountWithEnoughMoney() {
+        Account account = bank.getAccount(accountNumber);
+
+        assertEquals(amount, account.getMoney());
+    }
+
+    @Test
+    public void shouldFailToGetNonExistingAccount() {
+        IllegalStateException exception = assertThrows(IllegalStateException.class, () -> bank.getAccount(550));
+        assertEquals("account does not exist", exception.getMessage());
+    }
+}