Sexual desire is the sum of the forces that lean us toward and away from sexual behavior. The ordinary spectrum of sexual desire's intensity ranges between aversion, disinclination, indifference, interest, need, and passion. Although many individuals have a characteristic pattern of desire throughout their adult lives, this spectrum evolves considerably over the life cycle. It is clinically useful to think of desire as consisting of drive (biological), motive (individual and relationship psychology), and wish (cultural) components. Four master variables--age, gender, social situation, and health--affect sexual desire so basically that many remain blind to their presence. Five dark paradoxes of desire envelop most people at some time in their privacy: (1) Drive and motive are not in sync; (2) Behavioral fidelity is often associated with longing for infidelity; (3) Despite moral proscriptions, lust is possible; (4) Familiarity diminishes sexual interest; and (5) Derogation enhances sexual expression. Clinicians cannot afford to simplify desire's inherent contradictions. Researchers must simplify desire in order to measure it. In both the clinician's and the researcher's hands, sexual desire is a slippery concept.