Genetic factors have been estimated to affect the development of epilepsy in as many as 40% of the patients. Several genes have been identified underlying rare epileptic syndromes. Most epilepsies are most likely caused by a combined effect of several genes and environmental factors, and the genetic background of these diseases remains to be largely unknown. In practice, DNA diagnostics is possible only in a small proportion of epilepsy patients. In some cases gene testing is, however, an essential tool in diagnosis, e.g. facilitating the planning of pharmacological therapy for the patient's epilepsy.