This lecture provides a general overview on endometriosis and focuses mainly on its physiopathology. It also outlines the need for a classification. Dr. Ussia gives a very complete summary of the different attempts made to obtain an ideal classification and comments upon the deficiencies of the current ones. She proposes a new one based mainly on the anatomical localization and depth of penetration to divide it into four sub-classes that can be categorized depending on the extension. This classification looks simple and has already been published, but it has to be validated with further studies.