My two-essay dissertation revolves around understanding the financial crisis of 2008. First I focus on the repo market, a major funding source of the shadow banking system, and show the repo market can create and amplify the fragility of the system. Then I investigate a broader economy with heterogeneous agents and demonstrate how the dynamics of equilibrium asset prices and wealth distributions are determined. In Essay 1, I develop a dynamic model of collateral circulation in a repo market, where a continuum of institutions borrow from and lend to one another against illiquid collateral. The model emphasizes an important tradeo. On one hand, easier collateral circulation makes repos liquid and increases steady state invest-ment through sev...