The paper first reviews the conceptual framework underlying different measures of structural unemployment as well as alternative empirical methods that have been used to provide estimates of them. Drawing on this review, it goes on to develop a method for estimating time-varying NAIRUs across a range of OECD countries using a Kalman filter. It then discusses the resulting econometric estimates, and the scope for their further refinement given the associated range of uncertainties. Recent trends in the NAIRU estimates are reviewed: they fell in many countries in the second half of the 1990s, although actual unemployment has remained well above the NAIRU for a majority of countries throughout much of the 1990s, particularly in Europe. Finally, the relevance of such measures to analysing inflation developments and monetary policy is discussed.