Is there a role for central bank balance sheet policies away from the effective lower bound on interest rates? We extend the canonical DSGE model with financial frictions to include a fully specified central bank balance sheet. We find that the balance sheet size and composition can play a macroprudential role in improving the efficacy of monetary policy. The optimal balance-sheet policy aims at affecting duration risk held by banks in order to increase their resilience to shocks. Optimal short-run balance sheet policies bring no additional advantage to using the policy rate alone provided the optimal long-run balance sheet is already in place. Our results also highlight a key role for government debt maturity and bank regulation in determining optimal central bank balance sheets.