Endometriosis (ectopic endometrial tissue) is a benign condition with many symptoms (e.g., infertility dysmenorrhoea, pelvic pain) and may lead to serious complications. It affects women of reproductive age, however, in rare cases it may also develop before menarche. The estimated frequency of endometriosis used to be 1-2%, but since the advent of laparoscopy it has increased to 6-10%. Direct spread (e.g., in endometriosis interna seu adenomyosis), retrograde menstruation, embolisation, metaplasia together with defective immunological and genetic processes have all been confirmed as possible mechanisms of the development of disease. Laparoscopy plays a leading role in the diagnosis, however, physical examination and imaging techniques (e.g., sonography) remain an essential part of preoperative assessment. The treatments of endometriosis include various endocrine manipulations, surgery and palliation.