A perfect endometriosis classification should be a common language and an expert system which helps the gynaecologist to decide the treatment of each patient. The staging of endometriosis is not a new idea; the first classification based on histologic criteria was presented in 1941. Since this initial attempt, several classifications have been proposed. These various systems are reviewed in terms of their advantages and defects. The revised American Fertility Society (AFS) endometriosis classification system is now accepted worldwide as the endometriosis international language. Laparoscopic staging and measurement techniques are presented and discussed. In recent reports, changes to the revised AFS classification have been proposed, including a better description of atypical and deep infiltrating peritoneal implants, and a stage V for patients with bilateral, extensive, dense adhesions. Despite its well-known advantages, the revised AFS classification cannot be used as a satisfactory expert system. A better understanding of endometriosis is required to improve the present system. Scientifically based scores for each lesion and a marker for disease 'activity' will be fundamental to a classification which will act as a valuable expert system and become the endometriosis classification of the twenty-first century.