Endometriosis is a common condition which affects millions of women in the UK and happens when tissue similar to the womb lining, which is otherwise known as the endometrium, are found outside of the womb. Endometriosis can involve the lining of the pelvis and abdomen (peritoneum), in the ovaries, rectum and bladder plus many other areas such as the fallopian tubes. Rarely can it affect other organs such as the lungs. Endometriosis can be painful and can happen over a long time. Causing painful periods, Endometriosis can also result in a woman’s back and her pelvis being painful. It is unclear what causes endometriosis.