diff --git a/lib/matrix.rb b/lib/matrix.rb
index b39973b..ab4a899 100644
--- a/lib/matrix.rb
+++ b/lib/matrix.rb
@@ -1999,6 +1999,19 @@ class Vector
     self / n
   end
 
+  #
+  # Returns an angle with another vector. Result is within the [0...Math::PI].
+  #   Vector[1,0].angle_with(Vector[0,1])
+  #   # => 1.5707963267948966
+  #
+  def angle_with(v)
+    raise TypeError, "Expected a Vector, got a #{v.class}" unless v.is_a?(Vector)
+    Vector.Raise ErrDimensionMismatch if size != v.size
+    raise ZeroVectorError, "Can't get angle of zero vector" if magnitude == 0 || v.magnitude == 0
+
+    Math.acos( inner_product(v) / (magnitude * v.magnitude) )
+  end
+
   #--
   # CONVERTING
   #++
diff --git a/test/matrix/test_vector.rb b/test/matrix/test_vector.rb
index fa29496..ee278e0 100644
--- a/test/matrix/test_vector.rb
+++ b/test/matrix/test_vector.rb
@@ -169,4 +169,15 @@ class TestVector < Test::Unit::TestCase
     v = Vector[1, 0, 0].cross_product Vector[0, 1, 0]
     assert_equal(Vector[0, 0, 1], v)
   end
+
+  def test_angle_with
+    assert_in_epsilon(Math::PI, Vector[1, 0].angle_with(Vector[-1, 0]))
+    assert_in_epsilon(Math::PI/2, Vector[1, 0].angle_with(Vector[0, -1]))
+    assert_in_epsilon(Math::PI/4, Vector[2, 2].angle_with(Vector[0, 1]))
+    assert_in_delta(0.0, Vector[1, 1].angle_with(Vector[1, 1]), 0.00001)
+
+    assert_raise(Vector::ZeroVectorError) { Vector[1, 1].angle_with(Vector[0, 0]) }
+    assert_raise(Vector::ZeroVectorError) { Vector[0, 0].angle_with(Vector[1, 1]) }
+    assert_raise(Matrix::ErrDimensionMismatch) { Vector[1, 2, 3].angle_with(Vector[0, 1]) }
+  end
 end