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 (residual wage dispersion), different prices for same goods (residual price dispersion), the persistent effects of real and nominal shocks on employment and output. In this book, the author develops a serie s of models to analyze the macro-economic implications of public policies in the economies characterized by search frictions and their effects on unemployment, job-creation, labor market flows, and the dispersion of wages. He shows that frictions have important implications for efficiency, welfare, and public policy.