In recent years there has been a significant expansion in our understanding of the underlying etiologies of the epilepsies, underpinned by advances in modern neuroimaging and genetic testing. As such terminology such as 'idiopathic', 'cryptogenic' and 'symptomatic' are no longer used. Epilepsies are now described more precisely by their specific underlying etiologies. In this section of EpilepsyDiagnosis.org, important etiologies are presented, with particular focus on those that have distinct clinical, electrical or radiological features that are important to recognize and those for whom early recognition and treatment optimizes health outcomes.