Why do we do what we do? One commonsense answer is that actions follow ideas. We have ideas about who we are, what we believe, what we value, and how we ought to behave. These ideas-partly inherited and partly chosen, partly conscious and partly unconscious-structure our behavior. Much of what social scientists produce falls into the 'ideas' category. This includes well-worn subjects such as ideology, values, norms, roles, beliefs, religion, culture, political culture, as well as facets of identity captured by ethnicity, nation, race, gender, and sexuality. Yet within social science research, interests and institutions remain the preferred mode of explanation. In this ambitious and timely study of social behavior, John Gerring brings ideas into the fold of causal explanation. He offers an overview of the field, an in-depth discussion of the methodological obstacles facing ideational causes, and some potential solutions for researchers across social science fields.