Endometriosis is a disease characterized by the presence of endometriotic tissue outside the uterine cavity. Although its pathogenesis remains to be elucidated, immune status is suggested to play an important role in the initiation and the progression of the disease. In particular, immune cells in lymphoid lineage that comprised T and B lymphocytes and natural killer cells play essential roles in determining either accept or reject survival, implantation, and proliferation of endometrial and endometriotic cells. Numerous studies have shown aberrant functions of these immune cells in women with endometriosis. The abnormalities include reduced activity of cytotoxic T cells and NK cells, cytokine secretion by helper T cells, and autoantibody production by B lymphocytes. These alterations are suggested to be induced by various manners and promote the disease. Understanding of these immune aspects in endometriosis is thus expected to benefit the treatment of the disease.