Bilateral congenital cataract is genetic in at least 25% of cases. In contrast, unilateral congenital cataract is usually sporadic. Genetic heterogeneity is significant with the involvement of more than 30 genes having been identified to date. Phenotypes are defined by the location and morphology of the lens opacities. Mutations in the same gene may result in different phenotypes (clinical heterogeneity), and mutations in different genes may be associated with similar phenotypes (genetic heterogeneity). The mode of inheritance is mostly autosomal dominant but autosomal recessive and X-linked modes also occur. Expressivity may be variable and penetrance reduced. In X-linked cataract, carriers may show carrier signs. A precise pedigree analysis and a clinical examination of further family members are mandatory for correct genetic counselling. Metabolic cataract may be diagnosed biochemically. Molecular genetic analysis is not offered routinely to date with the exemption of a few genes.