Genetic factors are now recognised to have an even more important role in epilepsies than previously appreciated. Rare mendelian forms of epilepsy are now well recognised, and there is evidence of complex inheritance due to multiple susceptibility genes in most idiopathic epilepsies. The complexities of epilepsy classification and the variety of clinical genetic methodologies (family aggregation, twin, and multiplex family studies) have led to an apparently confusing picture. Molecular approaches have revealed genes for many mendelian epilepsies. Most encode ion-channel subunits, but major challenges remain in understanding phenotype-genotype relationships. These challenges are even greater in complex epilepsies in which gene discovery is still in its infancy. In this Review, we synthesise clinical genetic data, discuss the strengths and weaknesses of different approaches, and integrate molecular findings about the epilepsies. This knowledge not only informs clinicians about the biology of the epilepsies but also has important consequences for clinical practice and genetic counselling.