This collection of foundational papers on sex differences in the brain traces the development of a much-invoked, fast-growing young field at the intersection of brain and behavior. The reader is introduced to the meaning and nature of sexual dimorphisms, the mechanisms and consequences of steroid hormone action, and the impact of the field on interpretations of sexuality and gender. Building on each other in point-counterpoint fashion, the papers tell a fascinating story of an emerging science working out its core assumptions. Experimental and theoretical papers, woven together by editor's introductions, open a window onto knowledge in the making and a vigorous debate between reductionist and pluralist interpreters. Five major sections include papers on conceptual and methodological background, central nervous system dimorphisms, mechanisms for creating dimorphisms, dimorphisms and cognition, and dimorphisms and identity. Each section builds from basic concepts to early experiments, from experimental models to humans, and from molecules to mind. Papers by such leading scholars as Arthur Arnold, Frank Beach, Anne Fausto-Sterling, Patricia Goldman-Rakic, Doreen Kimura, Simon LeVay, Bruce McEwen, Michael Merzenich, Bertram O'Malley, Geoffrey Raisman, and Dick Swaab, illustrate a rich blend of perspectives, approaches, methods, and findings. Sex and the Brain will show students how a scientific paper can be analyzed from many perspectives, and supply them with critical tools for judging a rapidly emerging science in a contentious area. Gillian Einstein is Associate Professor in the Departments of Psychology and Public Health Science at the University of Toronto. She has held positions at Duke University and the National Institute of Health.