Endometriosis is a condition in which an endometrial-like tissue, which normally lines the uterus, grows outside of it, often in pelvic areas such as the ovaries, fallopian tubes, or peritoneum. This tissue can respond to hormonal fluctuations and bleed during the menstrual cycle, causing pain, inflammation, and adhesion formation. Symptoms include chronic pelvic pain, painful menstruation, and fertility problems.