This paper develops a model of banking fragility driven by aggregate liquidity shortages. Inefficiencies arise because liquidity smoothing across banks breaks down when there is such a shortage, causing unnecessary and value-reducing transfer of assets between banks. We find that a Lender of Last Resort policy is ineffective in restoring efficiency as it leads to offsetting changes in the banks’ supply of liquidity. In contrast, subsidizing the purchase of assets from troubled banks increases welfare by improving the banks’ liquidity holdings. The first best, however, is achieved by redistributing liquidity from healthy to troubled banks in a crisis.