This chapter provides detail description of etiopathology of endometriosis: Current thinking. Theory of celomic metaplasia and induction, hematogenous metastasis, of endometriosis, theory of lymphatic spread, genetic predisposition, theory of menstrual regurgitation, role of peritoneum, modification in immune function and endometriosis, adhesion of endometrial cells, angiogenesis of endometrial implant, role of endometrium in pathogenesis, endocrine function and endometriosis, oxidative stress and endometriosis, environmental factors are described in detail. 114 references are included at the end of the chapter to facilitate further reading.