This chapter provides an overview of how economics and sociology of conventions (EC/SC), or convention theory, has been applied to consumer behavior. Conventions can be defined as interpretive frameworks that consumers develop and mobilize along with producers to assess situations as well as to assert and justify particular consumption practices. Convention theory highlights two important points. (1) Quality is not an intrinsic property of products and services. It is evaluated by consumers according to criteria which may converge or diverge with those of producers. When these criteria diverge, signs of quality, standards, or simply a common definition of the situation may be necessary to restore a shared perception. When such conventions are not established, the very existence of the market or the access to the market for some categories of consumers may be jeopardized. (2) Consumers are reflexive and skilled when concerned with the moral dimensions of consumption. They formulate judgments on certain products, by mobilizing different orders of worth. Thus, rational calculation or knowledge can be less adequate to explain some specific consumer choices than the conventions these consumers apply or the relationships of trust they have with certain economic or social actors. This model of the orders of worth elucidates consumers’ critical competence and offers a framework for analyzing the multifaceted rationalities employed by consumers. In sum, convention theory appears well suited to improve the understanding of specific dimensions of consumption practices, such as the phenomenon known as the attitude-behavior gap in the field of sustainable consumption.