The subject of this book is reproduction-specifically, the interplay between reproductive physiology (especially neural and endocrine events) and behavior. In presenting this topic, there are two expository goals. The first is to study repro- duction at all of the major levels of biological organization-from the molecular (e. g. , hormone receptors in the brain), through the cellular (e. g. , ovarian morphogene- sis), systemic (e. g. , operation of the hypothalamo-pituitary-ovarian axis), and the organismic levels of organization. Analogously, behavior is treated from the most molecular, elementary, and fundamental components (e. g. , copulatory reflexes), through behavior in the reproductive dyad (e. g. , analysis of female sexual behav- ior), to complex social behavior (e. g. , the interaction of social context and behav- ioral sex differences). To the extent that these levels of biological and behavioral organization rep- resent a "vertical axis" in behavioral neurobiology, a second goal is to treat the "horizontal axis" of biological organization, viz. , time. There are, therefore, treat- ments of evolutionary origins (e. g. , a phylogenetic survey of psychosexual differ- entiation), genetic origins in the individual (e. g. , sexual organogenesis), ontoge- netic development (e. g. , behavioral sexual differentiation), and the immediate physiological precursors of behavior (e. g. , hormonal and nonhormonal initiation of maternal behavior). In addition to tracing the origins of reproduction and reproductive behavior, one extends the time-line from the behavior to its physio- logical consequences (e. g. , neuroendocrine consequences of sexual behavior).