The question of how sex differences in behavior among vertebrates emerge and are expressed has been the topic of intense study for over 50 years. Convergent evidence from birds and mammals, primarily rodents, has provided certain common principles while highlighting other species-specific properties. The importance of early hormonal effects on the developing brain to adult behavioral profile is pervasive throughout the vertebrate phyla and assures that brain sex phenotype will match gonadal phenotype. Variation in the magnitude of differences between males and females in sexual behavior, parenting and aggression are influenced by environmental and physiological parameters. Recent advances in the cellular and molecular mechanisms of steroid hormones in both organizing and activating neural circuits to control behavior reveal a wide variety of effector pathways and emphasize how much we have to learn.