Abstract: A window film anchoring device is disclosed which comprises a first engagement member and a second engagement member between which a free film portion of a film affixed to a window pane is inserted. The first and second engagement members are attached to a corresponding window frame portion, and film engaging surfaces of the first engagement member are in interlocking and lateral motion preventing relation with complementary film engaging surfaces of the second engagement member so as to maintain a constant pressing force on the free film portion. The anchoring device is adapted to be a retrofit kit, and is able to be easily attached to an existing window frame, in order to increase the impact or impulse resistance of the window pane. The anchoring device may also be factory assembled to a newly produced frame.