The sex therapy literature has concentrated on disorders of the excitement and orgasm phases of the sexual response. However, disorders of sexual desire have been virtually neglected, although low-libido disorders are highly prevalent, may be extremely distressful to patients and their partners, and influence the course and prognosis of therapy. This paper focuses on this important aspect of human sexuality. Some clinical features of hypoactive sexual desire are described, and some hypotheses about etiology and prognosis are presented.