The basic concepts of impact resistance are reviewed. Also discussed are the research on materials, treatment and testing procedures, mechanisms of fracture and the factors affecting impact resistance. Research has shown that the strength of a lens treated or untreated is well below the theoretical strength of glass. The impact resistance of a lens is altered by the presence of breaks in the surface of a lens. The drop ball test as mandated by the FDA is essentially a control test. Its use as a means of evaluating the performance of a lens has been open to question. Researchers have suggested alternative forms of testing which may be more valid in assessing the impact resistance of an ophthalmic lens.