This review of the literature on the 2007-2009 crisis discusses the precrisis conditions, the crisis triggers, the crisis events, the real effects, and the policy responses to the crisis. The precrisis conditions contributed to the housing price bubble and the subsequent price decline that led to a counterparty-risk crisis in which liquidity shrank due to insolvency concerns. The policy responses were influenced both by the initial belief that it was a market-wide liquidity crunch and the subsequent learning that insolvency risk was a major driver. I suggest directions for future research and possible regulatory changes.

We develop a theory of optimal bank leverage in which the benefit of debt in inducing loan monitoring is balanced against the benefit of equity in attenuating risk shifting. However, faced with socially costly correlated bank failures, regulators bail out creditors. Anticipation of this generates multiple equilibria, including one with systemic risk in which banks use excessive leverage to fund correlated, inefficiently risky loans. Limiting leverage and resolving both moral hazards—insufficient loan monitoring and asset substitution—requires a novel two-tiered capital requirement, including a “special capital account” that is unavailable to creditors upon failure.