The NAIRU can be regarded as an evolution of the natural unemployment rate concept. Both relate to the long-run vertical Phillips curve model and share the same macroeconomic properties, although some of the underlying mechanisms may differ according to the specific features of the analysis concerning the relation between unemployment and inflation. Empirical estimates of the NAIRU are very important for macroeconomic policy decision-making but at the same time have proved highly problematic in many respects, particularly in their dependence on actual unemployment paths. These difficulties have led to a revived interest in hysteresis but have not yet discarded the NAIRU concept and its policy role. An alternative perspective is briefly discussed.