Endometriosis is a common gynecologic disorder characterized by the presence of endometrial tissue outside the uterine cavity. Various theories have been put forth to explain the mechanisms for the development of this disease. Although no single theory can explain all cases of endometriosis, the retrograde menstruation theory has gained the widest acceptance. This theory proposes that viable endometrial tissue is refluxed through the fallopian tubes during menstruation and implants on peritoneal surface or pelvic organs. Retrograde menstruation occurs in 76% to 90% of women. The much lower prevalence of endometriosis suggests that additional factors determine susceptibility to endometriosis. Once in the peritoneal cavity, the survival and implantation of endometrial cells seem to be mediated by abnormal MMP and TIMP expression, altered immune milieu, aberrant local aromatase activity, and genetic and environmental factors.