Frictions in markets are all pervasive in real economies. They arise due to informational imperfections, heterogeneity among agents, the absence of perfect insurance markets, and limited mobility. The result is that trades in markets characterized by frictions are un-coordinated, time consuming, and costly. Models with frictions provide coherent explanations for many important features of real economies, which are not explained by models without frictions. These features include the use of money in transactions, the existence of unemployment, similar workers paid different wages...