The group character of delinquency has been recognized for more than 60 years, but the nature of delinquent groups remains poorly understood. Data from the National Survey of Youth are used to examine delinquent groups, with special attention to the identity and role of instigators in those groups. Delinquent groups are small and transitory, but 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 the other depending on their relative position in the group in which they are participating at the time. The roles that offenders adopt are thus determined, not by some stable individual trait, but by the situational interaction of group and individual characteristics.