Much of our knowledge about the function of genes in cataracts has been derived from the molecular analysis of spontaneous or induced mutations in the mouse. Mutations affecting the mouse lens can be identified easily by visual inspection, and a remarkable number of mutant lines have been characterized. In contrast to humans, most of the genetic mouse cataract models suffer from congenital cataracts, and only a few develop cataracts in old age. Therefore, the mouse cataract models contributed rather to the understanding of lens development than to the ageing process taking place in the lens. A prerequisite for molecular analysis is the chromosomal localization of the gene. In this review, several mouse models will be discussed with emphasis on the underlying genetic basis rather than the morphological features as exemplified by the following: (i) the most frequent mutations in congenital cataracts affect genes coding for gamma-crystallins (gene symbol: Cryg); (ii) some postnatal, progressive cataracts have been characterized by mutations in the beta-crystallin encoding genes (Cryb); (iii) mutations in genes coding for membrane proteins like MIP or connexins lead to congenital cataracts; (iv) mutations in genes coding for transcription factors such as FoxE3, Maf, Sox1, and Six5 cause cataracts; (v) mouse models suffering from hereditary age-related cataracts (e.g. Emory cataract) have not yet been characterized genetically. In conclusion, a broad variety of hereditary congenital cataracts are well understood at the molecular level. Further, expression patterns of the affected genes in several other tissues and organs outside the eye, is making it increasingly clear that isolated cataracts are the exception rather than the rule. By further understanding the pleiotropic effects of these genes, we might recognize cataracts as an easily visible biomarker for a number of systemic syndromes.