This paper presents a new perspective on the transition from early to middle childhood (i.e., human juvenility), investigated in an integrative evolutionary framework. Juvenility is a crucial life history stage, when social learning and interaction with peers become central developmental functions; here it is argued that the "juvenile transition" is a developmental switch point in the human life history, when both sex-related and individual differences in reproductive strategies are expressed after the assessment period provided by early childhood. Adrenarche, the secretion of adrenal androgens starting at the beginning of middle childhood, is proposed as the endocrine mechanism mediating the juvenile transition. It is argued that, in connection with the stress system, adrenal androgens enable adaptive plasticity in the development of reproductive strategies through integration of environmental and genetic factors. Finally, evidence is reviewed of both sex-related and individual differences arising during the juvenile transition, in the domains of attachment and aggression. Juvenility plays a central role in the ontogeny of behavior and personality; this paper contributes to defining its place within an integrated model of human development. (Contains 3 figures.)