In this paper, we investigate attitude-behaviour inconsistency as well as the effect of social norms and other peer-related factors on the translation of attitudes into behaviour. We draw upon social psychological perspectives, in particular from the social identity approach. Data were used from two waves of 1385 secondary school students in the School Study of the Netherlands Institute for the Study of Crime and Law Enforcement (NSCR). The results show that almost half of the respondents did not act in accordance with their previously measured attitudes on delinquency. Respondents who were exposed to an attitude-congruent social norm supportive of their own attitude reported significantly more attitude-consistent behaviour than respondents who were exposed to an incongruent norm. The effect of normative support was significantly moderated by attachment to peers but not by time spent with peers.