This is a topics course on ‘social interactions’, that is, socio-economic environments in which markets do not mediate all of agents’ choices, and each agent’s choice might be in part determined, for instance, by family, peer group, or ethnic group effects. The last decade has seen substantive work developed in that area that emphasized heterogeneity at the individual level so that the object of study is not a representative agent, and that the emergence of particular social behavior, norms, and group structures are outcomes of interaction at the individual level. This is a fascinating and exploding area of research that spans issues at the junction of sociology, psychology, and economics. We will see throughout how economists contribute to the discussion of social interactions, by bringing the economist’s toolbox to the table. Specific topics that we will cover include social norms, and crime.