This paper characterizes endogenous monetary policy when policymak-ers are uncertain about the extent to which movements in output and in-flation are due to changes in potential output or to cyclical demand and cost shocks. We refer to this informational limitation as the “information problem ” (IP). Main results of the paper are: 1. Policy is likely to be excessively loose (restrictive) for some time when there is a large decrease (increase) in potential output in comparison to a full information bench-mark. This provides a partial but unified explanation for the inflation of the seventies and the price stability of the nineties. 2. Errors in forecasting potential output and the output gap are generally serially correlated. 3. A quantitati...