diff --git a/billing/accounts.py b/billing/accounts.py
new file mode 100644
index 0000000000..e539f2f126
--- /dev/null
+++ b/billing/accounts.py
@@ -0,0 +1,30 @@
+class BankAccount:
+    def __init__(self, owner, balance=0.0):
+        self.owner = owner
+        if balance < 0:
+            raise ValueError("Initial balance cannot be negative")
+        self.balance = balance
+
+    def deposit(self, amount):
+        if amount <= 0:
+            raise ValueError("Deposit amount must be positive")
+        self.balance += amount
+        return self.balance
+
+    def withdraw(self, amount):
+        if amount <= 0:
+            raise ValueError("Withdrawal amount must be positive")
+        if amount > self.balance:
+            raise ValueError("Insufficient funds")
+        self.balance -= amount
+        return self.balance
+
+    def transfer(self, other_account, amount):
+        if not isinstance(other_account, BankAccount):
+            raise ValueError("Recipient must be a BankAccount instance")
+        self.withdraw(amount)
+        other_account.deposit(amount)
+        return self.balance, other_account.balance
+
+    def get_balance(self):
+        return self.balance
diff --git a/billing/tests/test_accounts.py b/billing/tests/test_accounts.py
new file mode 100644
index 0000000000..6960c5af93
--- /dev/null
+++ b/billing/tests/test_accounts.py
@@ -0,0 +1,61 @@
+import unittest
+
+from billing.accounts import BankAccount
+
+
+class TestBankAccount(unittest.TestCase):
+    def setUp(self):
+        self.account = BankAccount("John Doe", 100.0)
+
+    def test_init(self):
+        self.assertEqual(self.account.owner, "John Doe")
+        self.assertEqual(self.account.balance, 100.0)
+        with self.assertRaises(ValueError):
+            BankAccount("Jane Doe", -50.0)
+
+    def test_deposit(self):
+        new_balance = self.account.deposit(50.0)
+        self.assertEqual(new_balance, 150.0)
+        self.assertEqual(self.account.balance, 150.0)
+        with self.assertRaises(ValueError):
+            self.account.deposit(-10.0)
+        with self.assertRaises(ValueError):
+            self.account.deposit(0)
+
+    def test_withdraw(self):
+        new_balance = self.account.withdraw(50.0)
+        self.assertEqual(new_balance, 50.0)
+        self.assertEqual(self.account.balance, 50.0)
+        with self.assertRaises(ValueError):
+            self.account.withdraw(-10.0)
+        with self.assertRaises(ValueError):
+            self.account.withdraw(0)
+        with self.assertRaises(ValueError):
+            self.account.withdraw(1000.0)
+
+    def test_transfer(self):
+        other_account = BankAccount("Jane Doe", 50.0)
+        balances = self.account.transfer(other_account, 30.0)
+        self.assertEqual(balances, (70.0, 80.0))
+        self.assertEqual(self.account.balance, 70.0)
+        self.assertEqual(other_account.balance, 80.0)
+
+        with self.assertRaises(ValueError):
+            self.account.transfer(other_account, -10.0)
+        with self.assertRaises(ValueError):
+            self.account.transfer(other_account, 0)
+        with self.assertRaises(ValueError):
+            self.account.transfer(other_account, 1000.0)
+        with self.assertRaises(ValueError):
+            self.account.transfer("not an account", 10.0)
+
+    def test_get_balance(self):
+        self.assertEqual(self.account.get_balance(), 100.0)
+        self.account.deposit(50.0)
+        self.assertEqual(self.account.get_balance(), 150.0)
+        self.account.withdraw(30.0)
+        self.assertEqual(self.account.get_balance(), 120.0)
+
+
+if __name__ == "__main__":
+    unittest.main()