Unlike much previous research on sibling relationships, which emphasizes the congruence across various types of family experiences, the research described in this article explored between-family differences in patterns of experiences within families. The work is built upon Bronfenbrenner's ecological model and Magnusson's interactional perspective, as well as ideas from family sociology and social psychology. This article targets several themes emanating from the two main perspectives and shows how they have been applied in work on sibling relationships. Specifically, it considers the issue of sibling activities as important relationship phenomena, reviews work on contextual differences in children's sibling experiences, and describes ways of understanding patterns in children's sibling and family relationships. Two longitudinal studies form the basis for this discussion: the Penn State Family Relationships Project and the Penn State Study of Children's Everyday Experiences with Siblings. Contains 47 references. (SD)