diff --git a/src/math_utils.py b/src/math_utils.py
index e52f339..d05e908 100644
--- a/src/math_utils.py
+++ b/src/math_utils.py
@@ -9,3 +9,28 @@ def add(a, b):
 def multiply(a, b):
     """Multiply two numbers"""
     return a * b
+
+
+def fibonacci(n):
+    """Calculate the nth Fibonacci number (0-indexed).
+    
+    Args:
+        n: Non-negative integer
+        
+    Returns:
+        The nth Fibonacci number
+        
+    Raises:
+        ValueError: If n is negative
+    """
+    if n < 0:
+        raise ValueError("n must be non-negative")
+    if n == 0:
+        return 0
+    if n == 1:
+        return 1
+    
+    a, b = 0, 1
+    for _ in range(2, n + 1):
+        a, b = b, a + b
+    return b
diff --git a/tests/test_math_utils.py b/tests/test_math_utils.py
index 516f210..7055426 100644
--- a/tests/test_math_utils.py
+++ b/tests/test_math_utils.py
@@ -2,7 +2,7 @@
 from pathlib import Path
 sys.path.insert(0, str(Path(__file__).parent.parent / 'src'))
 
-from math_utils import add, multiply
+from math_utils import add, multiply, fibonacci
 
 def test_add():
     assert add(2, 3) == 5
@@ -13,3 +13,24 @@ def test_multiply():
     assert multiply(2, 3) == 6
     assert multiply(-1, 5) == -5
     assert multiply(0, 10) == 0
+
+
+def test_fibonacci_normal():
+    assert fibonacci(0) == 0
+    assert fibonacci(1) == 1
+    assert fibonacci(5) == 5
+    assert fibonacci(10) == 55
+
+
+def test_fibonacci_edge_cases():
+    assert fibonacci(2) == 1
+    assert fibonacci(3) == 2
+    assert fibonacci(4) == 3
+
+
+def test_fibonacci_negative():
+    try:
+        fibonacci(-1)
+        assert False, "Should have raised ValueError"
+    except ValueError as e:
+        assert str(e) == "n must be non-negative"