This dissertation is motivated by the housing crisis of 2008. It consists of three chapters. In the first chapter, Too Much Skin-in-the-Game? The Effect of Mortgage Market Concentration on Credit and House Prices, I propose a new theory to help explain the housing crisis. During the housing boom, a small number of institutions - the government-sponsored enterprises (GSEs) and a few banks - held most of U.S. mortgage risk. I develop a theory in which such concentration of mortgage exposure can explain features of the housing crisis. I show that large lenders with many outstanding mortgages have incentives to extend risky credit to prop up house prices. An increase in concentration can lead to a boom with worsening credit quality and a subs...