We investigated the relationship between group identification (with the family, school, and friendship groups) and adolescent health behaviour (smoking, binge drinking, and cannabis use). 1,111 students from 4 Scottish secondary (high) schools completed a questionnaire which included measures of group identification, group contact, health behaviours, and demographic variables. We found that identification with the family and school groups predicted reduced odds of substance use, whereas identification with the friend group predicted increased odds of substance use. Furthermore, the greater the number of social groups with which the participant strongly identified, the lower the odds that he/she participated in negative health behaviours. In contrast, merely having contact (rather than identifying strongly) with these groups increased the odds of participation in these behaviours. We suggest that group identification influences behaviour to the extent that it encourages adherence to group norms.