The vast majority of adolescents have at least one sibling, and most are raised by the same parent or parents. What then might researchers expect two adolescent siblings' representations of attachment to parents to be like? Are both siblings likely to exhibit similarly secure or insecure representations, or is it just as likely that one sibling would have a secure representation and the other an insecure one? Surprisingly, researchers know very little about whether adolescent siblings raised in the same family exhibit concordant representations of attachments to parents, yet the authors believe the answer to this question has important implications for attachment theory. The purpose of this chapter is to discuss this issue of sibling concordance during adolescence and its implications for attachment theory. (Contains 1 figure and 1 table.)