A substantial body of research suggests that, in spite of the risks associated with co-offending, offenders frequently choose to work together. Dominant theories of crime, as well as those of decisionmaking in general (e.g., game theory), typically assume that people's choices are based on instrumentally rational calculations; however, research on decisionmaking highlights the limitations of this assumption for understanding decisions to cooperate in both noncriminal and criminal activities. This article draws on social dilemma theories, as well as James Coleman's treatise on collective action (1990), for other insights into the motivation to cooperate. It extends these ideas and argues that the challenges of everyday adversity and contacts with potential co-offenders may increase people's willingness to trust others and co-offend. The article uses data from a two-wave panel of street youth in Toronto and Vancouver, testing these ideas in multivariate models of street theft. Of all the various styles examined, a form called collaboration was most important in facilitating crime. Figure, tables, notes, appendix, references