Lens development as a multistep process can be analyzed by the investigation of distinct cataract mutants. Since the mutant genes are molecularly characterized, the function of the wild-type allele can be deduced. Besides some mutations affecting the lens induction, which are not yet characterized at the molecular level, mainly mutations affecting the crystallin genes are discussed. In particular, for the murine γ-crystallin genes 8 different mutations are described in the mouse, which lead to different, distinguishable phenotypes. The distinct and complex phenotypes cannot be explained solely by the changed physicochemical properties of the altered crystallin packaging, but point to a regulatory function of the crystaliins during lenticular development and differentiation.