Introduction: the end of sex as we once knew it.

This special issue of Physiology and Behavior is devoted to papers summarizing sex differences that are now recognized to include not only reproductive behaviors, but also non-reproductive processes and phenomena such as feeding, thirst, pain, sensory processes, mood, cognitive function, the effects of stress, and the propensity for drug abuse. The purpose of this brief introduction is to trace some of the main themes and historical highlights in the discovery that the entire nervous system appears to be a target of reproductive hormones, as well as being subject to developmentally programmed sex differences.