Endometriosis is an often painful disorder in which tissue similar to the tissue that normally lines the inside of your uterus — the endometrium — grows outside your uterus. Endometriosis most commonly involves your ovaries, fallopian tubes and the tissue lining your pelvis. The causes of endometriosis are still unknown. One theory suggests that during menstruation, some of the tissue backs up through the fallopian tubes into the abdomen, a sort of "reverse menstruation," where it attaches and grows. Endometriosis can be a challenging condition to manage. An early diagnosis, a multidisciplinary medical team and an understanding of your diagnosis may result in better management of your symptoms.