The definition of the epileptogenic zone is a concept proposed by Jean Bancaud and Jean Talairach based on the anatomical, electrical and clinical correlations established from stereoelectroencephalographic recordings. They believed the epileptogenic zone to be the "region of the beginning and the primary organization" of ictal discharges. The opinion of North American authors is different: the epileptogenic zone is the "what to remove area" to produce freedom from seizures. This surgical definition assumes postsurgical validation. The aim of this paper is to show how to define the epileptogenic zone from all the stereoelectroencephalographic recording data.