What is endometriosis? Actually, it's where the endometrial tissue, which is the inside of the uterine lining, is not where it's supposed to be. It's not inside of the uterus, it's outside of the uterus and it can be in various places. It can be on the outside of the uterus itself. It could be on the ovary. It can be on the tubes. It can be on your bowel. It can be anywhere in the abdominal cavity. And actually it can even be in your skin.