This dissertation investigates two aspects of how to regulate the financial sector optimally in order to increase macroeconomic stability and mitigate the risk of future financial crises. Chapter 1 analyzes the desirability of international coordination in financial regulation. It develops a two-country model of systemic liquidity risk-taking in which financial market imperfections provide a rationale for macro-prudential regulation. In the model, curbing liquidity risk-taking via regulation lowers the price of liquidity during financial crises and thereby reduces the costs associated with market incompleteness. But regulation also entails costs in the form of distortions to productive investment decisions. The discrepancy between the dome...