The purpose of this study was to determine whether criminal thinking underpins peer influence and selection. It was predicted that proactive criminal thinking would mediate the peer influence effect (peers → offending) and reactive criminal thinking would mediate the peer selection effect (offending → peers). Participants were 1,170 male delinquent youth from the Pathways to Desistance study. The Moral Disengagement scale (proactive criminal thinking) and Peer Delinquent Behavior scale (peer delinquency) were cross-lagged to predict criminal offending, and the Weinberger Impulse Control scale (reactive criminal thinking) and criminal offending were cross-lagged to predict peer delinquency. Consistent with predictions, proactive but not reactive criminal thinking successfully mediated the peer → offending relationship and reactive but not proactive criminal thinking successfully mediated the offending → peer relationship. Whereas delinquent peer associations appear to promote proactive criminal thinking and peer influence, early criminal offending appears to promote reactive criminal thinking and peer selection.