Abstract: Concepts are the building blocks of legal doctrine. All legal rules and standards, in fact, are formed by combining concepts in different ways. But despite their centrality, legal concepts are not well understood. There is no agreement as to what makes a legal concept useful or ineffective—worth keeping or in need of revision. Social scientists, however, have developed a set of criteria for successful concepts. Of these, the most important is measurability: the ability, at least in principle, to assess a concept with data. In this Essay, we apply the social scientific criteria to a number of concepts and conceptual relationships in American constitutional law. We show that this field includes both poor and effective concepts and conceptual links. We also explain how the examples of poor concepts could be improved.