This thesis develops three models that study the motivation of various agents to take on debt, and the impact that excessive financial leverage can have on social welfare. In the chapter "Short-term Bank Leverage and the Value of Liquid Reserves", the incentive of a bank to take on leverage stems from the fact that its short-term debt provides liquidity benefits to depositors, at the cost of exposing the bank to the risk of runs. In the model, a bank possesses two instruments to manage the risk of runs: its funding policy and the size of its liquid asset holdings, modeled as government bonds. Issuing more debt allows the bank to better capture the liquidity benefits fits priced into deposits but increases illiquidity risk. Holding more bond...