Normally, the endometrium is located inside the uterine body, and when endometrial tissue appears outside the uterine body for various reasons, it is called endometriosis. Ectopic endometrium can invade any part of the body, including the umbilicus, bladder, kidneys, ureters, lungs, pleura, breasts, and even the arms and thighs. However, the vast majority are located in the pelvic organs and wall peritoneum, with the ovaries and uterosacral ligament being the most common, followed by the peritoneum of the uterus and other organs, and the vaginal rectal diaphragm. Ectopic in the ovary, can form ovarian chocolate cysts; ectopic in the myometrium, can form uterine adenomyosis and so on. The cause of endometriosis is not clear. It may be related to the reflux of menstrual blood or intrauterine surgery, the spread of the endometrium to other parts of the uterus through the venous or lymphatic system; it may also be related to genetic factors, immunity and inflammation, and other factors. When endometriosis occurs, about 25% of patients may be asymptomatic, and for those who are symptomatic, the symptoms are mostly related to the menstrual cycle. Common symptoms include lower abdominal pain and dysmenorrhea, typically secondary to dysmenorrhea, progressive aggravation, with pain located in the lower abdomen, lumbosacral and mid-pelvis, sometimes radiating to the perineum, anus, and thighs, which often occurs at the time of menstruation and lasts throughout the menstrual period. In addition, endometriosis can also cause infertility, with the infertility rate as high as 40%; discomfort during sexual intercourse, such as deep dyspareunia, which is most noticeable before the onset of menstruation; and menstrual abnormalities, such as increased menstrual flow, prolonged menstrual period, menstrual dripping, or premenstrual spotting bleeding, and so on. To summarize, endometriosis is caused by the endometrium located in other parts of the body, mainly manifested as lower abdominal pain, dysmenorrhea, infertility, discomfort during sexual intercourse, menstrual abnormalities and so on.