Although research on sibling relationships has been far less frequent than research on other close relationships such as parent–child, peer, and romantic partner relationships, researchers have found siblings to be important for the development of social competence as well as positive and negative adjustment. In addition, the sibling relationship is considered the longest lasting relationship across the life span and it serves unique developmental functions. This essay briefly describes foundational research on the influence of dyadic structural variables, relationship dynamics, and sibling influences on adjustment; outlines cutting-edge research within the field on the contexts of family ethnicity, developmental period, and important processes and influences on relationship dynamics; and discusses key issues for future research such as expanding to under-studied ethnic groups (e.g., Native American and Asian-American families), family structures and contexts (e.g., adoption, single-parents by choice, gay/lesbian parents), and mechanisms for relationship influence. Expanding the field to incorporate such research questions will likely require sibling researchers to examine findings from research on other important, close relationships, as well as collaboration of researchers from a variety of psychological disciplines as well as in the fields of sociology, neuroscience, genetics, anthropology, and human development and family studies.