The goals of the two studies reported here were to examine the nature of sibling typologies and to describe a new measure designed to classify the type of sibling relationship. In the first study, participants (N= 172) ranging in age from 18 to 65 years described their current sibling relationships using a new measure (Sibling Type Questionnaire; STQ). Principal components analysis reduced these data to five dimensions (mutuality, criticism, apathy, competition, and longing), which were used in a cluster analysis to partition the sample into five groups (supportive, longing, competitive, apathetic, and hostile). The second study was conducted to validate the classifications provided by the STQ by obtaining descriptions of sibling relationships from a representative sample of adults (N= 658) ranging in age from 18 to 86 years. Factorial congruence was established with each of the five dimensions previously identified, and cluster analyses again were employed to partition the sample into supportive, longing, apathetic, hostile, and competitive groups. Respondents completed the Adult Sibling Relationship Questionnaire (ASRQ) and a brief interview to provide a means of validating the cluster solution. Differences in the nature of sibling relationships associated with phases of adult life were also assessed.