This chapter reviews existing theory and empirical evidence on changes in co-offending patterns over the life course, links these patterns to other key criminal career parameters, and highlights important areas for future research. In order to set the stage for the remainder of the chapter, the few theoretical insights that relate joint offending to the development of criminal careers are first reviewed. The chapter then focuses on the age–co-offending curve, outlining what is known about changes in both co-offending prevalence and form across the life course and considering implications for more general developmental and life-course discussions of age and offending. It next examines how co-offending is related to other features of the criminal career, including onset, specialization, seriousness, frequency, duration, persistence, and desistance. Finally, this chapter briefly outlines outstanding issues and next steps for advancing DLC theory and research on group crime.