Endometriosis is a disorder where tissue (endometrium) that is supposed to grow inside the uterus will grow outside the uterus. It affects the fallopian tubes, the tissue lining the pelvis, and ovaries. This extraneous tissue behaves much the same way it would if it were in the uterus: thickening, breaking down, and bleeding with every cycle. This makes endometriosis very painful. There are often also issues with fertility. Fortunately, there are many treatments now, although no cure.