Although siblings act as socializing agents in cultures throughout the world (Weisner & Gallimore, 1977), historical and theoretical considerations have led many Western researchers to ignore or underestimate the positive contribution of siblings to the socialization of younger family members (Zukow, Chapter 1, this volume). Although the family is recognized as the matrix in which socialization first occurs, I will argue that siblings as well as adults engage in this process with younger family members. To assess the contribution of siblings to the socialization of younger siblings from a fresh perspective, I will address the social-interactive and perceptual processes underlying socialization. From this perspective, socialization is a process of co-construction in which caregivers and novice members are continuously building up and breaking down the social world from the ebb and flow of perceptual information. The degree to which siblings embody the role of socializing agent will be examined by evaluating evidence from Western and Third World studies. The discussion will consider the relative value, effectiveness, and uniqueness of socialization by siblings, including its impact on the direction of future research and the shape of emerging theories of socialization.