This issue of Neuroendocrine Perspectives, Volume 9, begins with a fascinating aspect of neurobiology: the steroid control of vertebrate behavior, in this case aggressive behavior. In the first chapter, Schlinger and Callard, after an extensive historical perspective of the behavioral effects of steroid hormones, review results of their own and others' laboratories on the hormonal basis of individual and sex-related differences in the intensity ofaggressive displays in birds. These studies provide evidence that brain aromatase exerts considerable control over both sex-dependent and individual...