Endometriosis is the presence of endometrial tissue outside both the uterine cavity and myometrium. Adenomyosis is endometrial tissue found within the myometrium. These two conditions have different clinical features and should be viewed as distinctly different entities. Clinicians have been fascinated by endometriosis since Rokitansky first described it in 1860. It arouses interest because endometriosis is one of the commonest gynecological conditions and is responsible for considerable morbidity. Despite this, however, we are still unclear as to its etiology and treating the disease can often be extremely challenging. This chapter aims to give the most up-to-date facts about endometriosis and adenomyosis. Special attention should be paid to the pathophysiology of endometriosis because understanding this enables the clinician to deduce the possible symptoms and complications of the disease. Emphasis has been placed upon the principles behind managing endometriosis. A flow chart of this is included to aid recall. The drugs used are described in detail, as clinicians use these regularly