Data show that delinquent groups are small and transitory but that offenders commonly belong to multiple groups and thus have a larger network or pool of accomplices. Groups appear to be more specialized than individuals, which suggests that offense specialization is the primary source of group differentiation. Most delinquent groups have an identifiable instigator, a person who tends to be older, more experienced, and emotionally close to other members. Males almost always follow other males, whereas females are much more likely to follow a member of the opposite sex. As a rule, offenders do not consistently assume the role of instigator or joiner over time, but instead switch from one role to another, depending on their relative position in the group in which they are participating at the time. Roles offenders adopt are thus determined by the situational interaction of group and individual characteristics rather than by some stable individual trait. 33 references, 5 tables, and 2 figures