The ratio of individual to co-offenders varies among crimes and age groups. Solo offending criminal careers are less common than those of exclusively co-offending, but the typical criminal career is a mix of offenses committed alone and with others. Co-offending is more characteristic of juvenile than adult criminality. Distinction must be made between co-offending involving gangs, groups, and criminal networks. Most delinquent groups are unstable. Desistance from co-offending results from transience, from the maturing of group members, and from the effects of intervention. Accomplice relationships are short-lived, and active co-offenders tend to have many different accomplices. Intervention efforts may succeed especially when focused on individuals who are both high-rate offenders and active recruiters to delinquent groups and for specific crimes. An increased understanding of co-offending, recruitment, and desistance and the implications of these processes for crime control policies can be gained from prospective, longitudinal cohort studies that include the experiences of co-offenders of cohort members. 3 tables and 100 references. (Author abstract modified)