Chapter 19. Market Liquidity—Theory and Empirical Evidence In this paper we survey the theoretical and empirical literatures on market liquidity. We organize both literatures around three basic questions: (a) how to measure illiquidity (b) how illiquidity relates to underlying market imperfections and other asset characteristics and (c) how illiquidity affects expected asset returns. Using a unified model from Vayanos and Wang (2010) we survey theoretical work on six main imperfections: participation costs transaction costs asymmetric information imperfect competition funding constraints and search—and for each imperfection we address the three basic questions within that model. We review the empirical literature through the lens of the theory using the theory to both interpret existing results and suggest new tests and analysis.