Since the initial description of endometriosis, several histogenetic theories have been put forward. Endometriosis has been attributed either to coelomic metaplasia, or to embryonic remnants or to the dissemination by various routes and subsequent grafting of endometrial fragments; this would explain the different anatomical locations of the disease as found clinically. Several contributing factors are also involved, including menstrual reflux or, more hypothetically, disturbances of local hormonal conditions, familial and genetic factors and possibly immune disorders. The role played by these factors has been demonstrated by clinical, epidemiological or experimental studies. Owing to our knowledge of histogenesis and of the aetiological factors involved in the development of endometriosis, the risk factors of the disease and its natural history are now better understood, and a better approach to its treatment will perhaps be found in the near future.