This article explores the relationship between norms and action, making a distinction between social and moral norms, quasi-moral norms, legal norms, and conventions. It defines ‘social norm’ as a non-outcome-oriented injunction to act or to abstain from acting, sustained by the sanctions that others apply to norm violators. After explaining how social norms operate, the article considers some important or representative social norms, including work norms, tipping norms, queueing norms, fairness norms and political norms. It also compares social norms with other motivations to act and argues that social norms are ultimately sustained by the emotions of contempt (or indignation) and shame. Yet the influence of emotion on behavior is much larger than the impact mediated by social norms.