Objective: Social norms are a key determinant of young adult drinking, yet little research has evaluated potential interactive effects among different types of norms. The present research was designed to evaluate perceptions of friends' approval of drinking (i.e., injunctive norms) as a moderator of the relationship between perceived prevalence of friends' drinking (i.e., descriptive norms) and personal alcohol consumption. We also evaluated whether social drinking motives further influence this relationship. Method: Participants included 1,400 first-year college students (61% women) who completed Web-based assessments of descriptive and injunctive norms, personal drinking, and social drinking motives. Results: Results revealed that both descriptive and injunctive norms regarding close friends were uniquely and positively associated with drinking behavior. The relationship between perceived descriptive norms and personal alcohol consumption was stronger among those who also perceived their friends as being more approving of drinking but only among students who reported stronger social motives for drinking. Conclusions: Descriptive and injunctive norms are distinct constructs and are not interchangeable, having both unique and interactive effects, and vary as a function of social motives. Results are considered in terms of their implications for brief interventions.