Endometriosis, defined by the presence of endometrial tissue outside the uterine cavity, is a common condition affecting 10% of women in the reproductive age. Menstrual factors reported to increase risk include dysmenorrhea, early menarche, and shorter cycle lengths. The theory of retrograde menstruation with implantation of endometrial fragments, in conjunction with peritoneal factors to stimulate cell growth is the most widely accepted. There is a growing body of evidence that immunological factors and angiogenesis play a key role in the pathogenesis of endometriosis. In women with endometriosis, there appears to be an alteration in the function of peritoneal macrophages, natural killer cells and lymphocytes, with production of growth factors and inflammatory mediators in the peritoneal fluid. Survival, adhesion, proliferation, invasion and vascularization of endometrial tissue in abdominal cavity may be the consequence of retrograde menstruation and referred to as implantation theory.