Keith M. Carlson, in the fourth article in this Review, examines the question, How much lower can the employment rate go without accelerating inflation? This critical rate, usually referred to as the “natural rate of unemployment,” was the subject of major research in the late 1970s. At that time, estimates of the natural rate ranged between 5 percent and 7 percent, but generally centered on 6 percent. Carlson reviews the developments that have affected the natural rate since World War II, focusing on the period since 1979. The author notes that the most obvious structural change in recent years has been the shifting age composition of the labor force, which has reduced the natural rate about one-half percentage point below its 1979 level. The minimum wage, individual and employer tax rates and, possibly, unemployment benefits also have moved favorably. As a result, the current natural rate appears to be well below the 6 percent midpoint estimated in 1979.