Chapter 5. Imperfect Information and Aggregate Supply This paper surveys the research in the past decade on imperfect information models of aggregate supply and the Phillips curve. This new work has emphasized that information is dispersed and disseminates slowly across a population of agents who strategically interact in their use of information. We discuss the foundations on which models of aggregate supply rest as well as the microfoundations for two classes of imperfect information models: models with partial information where agents observe economic conditions with noise and models with delayed information where they observe economic conditions with a lag. We derive the implications of these two classes of models for the existence of a nonvertical aggregate supply the persistence of the real effects of monetary policy the difference between idiosyncratic and aggregate shocks the dynamics of disagreement and the role of transparency in policy. Finally we present some of the topics on the research frontier in this area.