In this paper we measure potential output (and consequently the output gap) using state-space models. Given that the estimated output gap is used as an indicator to measure the extent of inflationary pressures in the economy, we evaluate the use of such models for the implementation of monetary policy. Our starting point is the Gerlach and Smets (1997) unobserved-components model, which they applied to the G7 countries. After subjecting this model to various diagnostic tests, we modify certain assumptions in it to reflect specific aspects of the Canadian economy. In particular, we focus on the specification of the permanent component of output and of inflation expectations, the issue of whether to use core or total inflation in the model, and the integration of appropriate supply shocks in the Phillips curve. In each case, the model is subjected to diagnostic tests and is examined for its out-of-sample forecasting performance. With the various modifications, we find that misspecification is somewhat alleviated and out-of-sample forecast performance is improved. Based on this performance, we feel that state-space models of the output gap can be quite useful in the formulation of monetary policy.