WILL POTENTIAL OUTPUT GROW in the future at a 4 percent annual rate, as several of the more optimistic business economists assume, or at the pathetic 1.8 percent annual rate assumed into the distant future by the trustees of the Social Security Administration? (1) Put differently, will real GDP in seventy-five years be 20 times its current level or a mere 3 1/2 times? Academic research on future supply-side issues has focused mainly on the causes of the post- 1995 productivity growth revival, but the growth rate of potential output is of independent interest. Variations in four factors--population growth, labor force participation, the unemployment rate, and hours worked per employee--can create significant differences between the long-run path of potential output and that of trend productivity growth. These differences matter for numerous issues, including long-run fiscal policy, the solvency of entitlement programs, the balance of world saving and investment, and the role of the United States as an engine of growth for the rest of the world. This paper has three goals. The first is to forecast growth in U.S. potential real GDP, not for the full seventy-five-year horizon of the Social Security trustees, but for the more modest but still daunting span of the next two decades. It brings together recent research both about productivity and about the likely future behavior of the other four factors, especially population growth, that matter for potential output growth. The need to predict future population growth in turn requires an exploration of the determinants of trends in fertility and mortality rates, as well as the likely future trend of net immigration into the United States. The second goal of the paper, connected closely with the first, is to interpret the extraordinary productivity performance of the United States since 1995 and especially since mid-2000. Far from slowing in response to the 2001 recession and the collapse of investment in information and communications technology (ICT) after mid-2000, growth in labor productivity actually accelerated from an average of 2.56 percent a year between 1995:4 and 2000:2 to 3.46 percent a year between 2000:2 and 2003:2. Should a forecast of future productivity growth use as its precedent the average behavior of actual productivity growth over the past two years, the past eight years, or some longer interval? The third goal of the paper, related to the first two, is to provide a new breakdown of past U.S. economic growth into its trend and cyclical components. In assessing long-term growth performance over some historical period, one would not want to include the portion of real GDP growth contributed by a sharp difference in cyclical conditions, for example between the 7.6 percent unemployment rate of mid-1992 and the 4.0 percent rate of early 2000. This paper bases its cyclical analysis on an identity that links real GDP to productivity, the employment rate, and several other variables. This analysis uncovers important changes in cyclical behavior between the earlier postwar downturns and the two recent jobless recessions and recoveries (1990-93 and 2001-03). One particularly important difference is the strength of productivity growth and the weakness of payroll employment growth in both of the most recent episodes and especially the latest. To predict the future, one naturally starts with the past. But how much of the past? This is not a question on which either economic or statistical theory provides much guidance. Productivity growth was much faster after 1995 than between 1972 and 1995, but if one is trying to make forecasts twenty or more years into the future, the slow-growth period has at least some relevance, as do the more rapid growth periods 1950-72 and 1995-2003. Sometimes one has good economic reasons for looking at only part of history, believing strongly, for instance, that the economic dislocations of the Great Depression and World War II will not recur. …