Endometriosis is a common disorder, yet it is among the most enigmatic of gynaecologic diseases, classically defined as the presence of endometrial glands and stroma outside the uterine cavity and musculature. It is unclear whether both glands and stroma are required for the pathological definition of the disease. Endometriosis is different from the presence of endometrial tissue within the uterine musculature, a pathology termed adenomyosis. A longer article on recent insights in histogenesis, epidemiology, clinical presentation (infertility,pelvic pain), pathology and diagnosis of endometriosis can be found in the New England Journal of Medicine, 328 (1993), pag. 1759-1769.