Seller reputation is an important asset because buyers often choose sellers on the basis of their reputation. This is particularly true when the quality of the good or service transacted is hard to measure and the parties cannot perfectly contract on the outcome of the transaction. As a consequence, the seller will be mindfulof building and maintaining a good reputation through the information that buyers have about the seller, including previous transactions and the reports of other buyers. We introduce a unifying framework that embeds a number of different approaches to seller reputation, incorporating both hidden information and hidden action. We use this framework to stress that the way in which consumers learn affects both behavior and outcomes. In particular, the extent to which information is generated andsocially aggregated determines the efficiency of markets. After reviewing these theoretical building blocks we discuss several applications and empirical concerns. We highlight that the environment in which a transactionis embedded can help determine whether the transaction will occur and how parties will behave. Institutions, ranging from the design of online markets to norms in a community, can be understood as ensuring that concerns for reputation lead to more efficient outcomes. Similarly, the desire to affect consumer beliefs regarding thefirm’s incentives can help us understand strategic firm decisions that seem unrelated to the particular transactions they wish to promote. We conclude by considering slightly different models of reputation that lie beyond the scope of our framework, briefly reviewing the somewhat sparse empirical literature, and highlighting and suggesting future directions for research.