The 1959 publication of "Organizing action of prenatally administered testosterone propionate on the tissues mediating mating behavior in the female guinea pig" by Charles H. Phoenix, Robert W. Goy, Arnold A. Gerall, and William C. Young transformed how sex differences in mating behavior were thought to develop. Previous work provided extensive evidence that steroid hormones activated patterns of male and female sexual behavior, but only activated the behavioral patterns typical of a given sex. The 1959 paper explained this phenomenon by arguing that androgens, or their metabolites, acting at specific time(s) during development sexually dimorphically organized the tissues mediating mating behavior, which were activated by appropriate hormonal stimulation in adulthood. Thus, exposure to steroids at specific time(s) permanently altered the structure or function of the organism. The exact hormone, exact timing, exact mechanism, and exact tissues were unspecified in the article. The last two paragraphs of the discussion illustrate the investigators' unresolved views. The first proposes that the 'organization' was likely to be functional and not evident in visible structure, whereas the next paragraph argues that behavioral change implies structural change and thus structural changes are the likely consequence of steroid actions. These unresolved issues have produced extensive work in the intervening 50 years. The papers in this issue mark the 50th anniversary of this landmark paper and reflect the scope and relevance of the issues raised in the original paper and demonstrate the progress that has been made in understanding the Organizational Hypothesis and its impact on sexual differentiation.