diff --git a/bank.py b/bank.py
index ab74f6f..f108c9d 100644
--- a/bank.py
+++ b/bank.py
@@ -81,7 +81,12 @@ def get_transaction_history(self):
     
     def apply_interest(self, rate):
         # Applies interest based on a given rate.
-        pass
+        if rate < 0:
+            return False
+        interest = self.balance * rate
+        self.balance += interest
+        self.transactions.append(("Interest", interest))
+        return True
 
     
     def overdraft_protection(self, amount):