In this introductory chapter, a classification of the etiology of epilepsy is proposed, and this forms the basis of the sectional divisions in the rest of the book. In constructing such a classification, it is necessary to take cognisance of five particular points: Definitions, Multifactorial cause of epilepsy, Cause versus mechanism, Focal versus generalized epilepsy and Flexibility. The chapter presents the definitions of the epilepsies which are divided into four main categories: Idiopathic epilepsy, Symptomatic epilepsy, Provoked epilepsy and Cryptogenic epilepsy. The ILAE Commission recommended that a classification should be a database forming the basis of a diagnostic manual; the etological schema in this chapter should be viewed as such for instance in relation to the benign focal epilepsies or even the idiopathic generalized epilepsies. It is clear that in the future, as further knowledge accrues, some of these epilepsies may be reclassified and revised.