diff --git a/libs/geom/include/psemek/geom/quaternion.hpp b/libs/geom/include/psemek/geom/quaternion.hpp
index 12afb858..606b72b3 100644
--- a/libs/geom/include/psemek/geom/quaternion.hpp
+++ b/libs/geom/include/psemek/geom/quaternion.hpp
@@ -28,6 +28,9 @@ namespace psemek::geom
 
 		T & operator[] (std::size_t i) { return coords[i]; }
 		T const & operator[] (std::size_t i) const { return coords[i]; }
+
+		quaternion<T> & operator += (quaternion<T> const & other);
+		quaternion<T> & operator -= (quaternion<T> const & other);
 	};
 
 	template <typename ... Args>
@@ -136,6 +139,20 @@ namespace psemek::geom
 		return r;
 	}
 
+	template <typename T>
+	quaternion<T> & quaternion<T>::operator += (quaternion<T> const & other)
+	{
+		(*this) = (*this) + other;
+		return *this;
+	}
+
+	template <typename T>
+	quaternion<T> & quaternion<T>::operator -= (quaternion<T> const & other)
+	{
+		(*this) = (*this) - other;
+		return *this;
+	}
+
 	template <typename T>
 	T norm_sqr(quaternion<T> const & q)
 	{