Deeply infiltrating endometriosis can be defined as endometriosis infiltrating deeper than 5 mm under the peritoneal surface. Type I is a conical lesion suggested to be caused by infiltration; type II is mainly caused by retraction of the bowel over the lesion; type III is the most severe lesion suggested to be caused by adenomyosis externa. Severe cases are clinically apparent by nodularities in the pouch of Douglas, whereas mild and subtle forms of deep endometriosis are easily missed. Clinical examination during menstruation and scrutiny at laparoscopy for indurations, followed, preferably, by CO2-laser-excision are the key features for diagnosis and treatment. It is important to realize that depth of infiltration and lateral spread cannot be evaluated by laparoscopic inspection but only during excision, that CA125 concentration but not ultrasound or nuclear magnetic resonance can be helpful in the diagnosis, and that in the most severe cases medical pretreatment is advocated. Results of excision, as evaluated by disappearance of pain in some 80% of women, by a cumulative pregnancy of some 70% and a low recurrence rate, are excellent. The peritoneal fluid is thought to play a key role in the physiopathology of deep endometriosis which is considered to be endometriosis which has escaped from the influence of the peritoneal fluid. This concept is clinically important for the medical treatment of endometriosis, which is suggested to shrink deep lesions and to bring them back under peritoneal fluid control. A model of endometriosis is proposed and discussed. Subtle lesions are considered a natural condition occurring intermittently in all women, whereas we question whether mild endometriosis is a disease. In some women endometriosis has an aggressive behavior and develops into cystic ovarian endometriosis or into deeply infiltrating endometriosis. In this model subtle and mild forms would be called "endometriosis," whereas deep and cystic ovarian forms could be called "endometriotic disease." It is stressed that deep and cystic ovarian endometriosis are two distinct entities, which is important for our understanding of endometriosis, for classification and for treatment of endometriosis.