This review surveys the field of women's mental health, with particular emphasis on its evolution into a distinct area of biomedical research. The field employs a biomedical disease model but it also emphasizes social and cultural influences on health outcomes. In recent years, its scope has expanded beyond studies of disorders occurring in women at times of reproductive transitions and it now encompasses a broader study of sex and gender differences. Historical and conceptual influences on the field are discussed. The review also surveys gender differences in the prevalence and clinical manifestations of mental disorders. Epidemiological findings have provided a rich resource for theory development, but without research tools to test theories adequately, findings of gender differences have begged the question of their biological, social, and cultural origins. Clinical depression is used to exemplify the usefulness of a sex/gender perspective in understanding mental illness; and major theories proposed to account for gender differences are critically evaluated. The National Institutes of Health (NIH) is the primary federal funding source for biomedical women's mental health research. The review surveys areas of emphasis in women's mental health research at the NIH as well as some collaborative activities that represent efforts to translate research findings into the public health and services arenas. As new analytic methods become available, it is anticipated that a more fundamental understanding of the biological and behavioral mechanisms underlying sex and gender differences in mental illness will emerge. Nonetheless, it is also likely that integration of findings predicated on different conceptual models of the nature and causes of mental illness will remain a challenge. These issues are discussed with reference to their impact on the field of women's mental health research.