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Sex steroids play important and diverse roles in the regulation of structure and function of the central nervous system. Early in life, steroids shape the structure of sensitive areas of the brain, especially those involved in the control of reproductive behavior and ovarian function. Original studies demonstrating organizing effects of steroids on the brain were carried out in rodents, but more recently these studies have been extended to primates, including humans. Throughout life, sex steroids regulate neural function by influencing steroid receptor-bearing neurons and by influencing neurons via steroid receptor-independent mechanisms. Sex steroid receptors have been identified in the brain, especially in the phylogenetically ancient structures that regulate reproductive behavior. Sex steroids that affect neural function can originate peripherally from the brain and/or adrenal gland, and can be synthesized within the brain itself. A number of neurally active progestogens and androgens are synthesized de novo in the brain, and estrogens can be converted within the brain from androgens by the enzyme aromatase. Thus, ovarian and central nervous system sex steroids play important roles in regulating reproductive behavior by regulating neural structure and function.