In response to the …nancial crisis of 2007/08, all major central banks decreased interest rates to historically low levels and created large excess reserves. Central bankers and academics currently discuss how to implement monetary policy, going forward. We …nd that paying interest on reserves (IOR) is optimal if the central bank has full …scal support. If the central bank has no …scal support, reducing reserves is optimal. This can be achieved by reserveabsorbing operations which hold the size of the balance sheet constant, or by selling assets which reduces the size of the balance sheet.