It is now 25 years since the growth rate of labor productivity and of multi-factor productivity (MFP) decelerated sharply both in the United States and in most other industrialized nations. This ‘‘productivity slowdown’’ has eluded many attempts to provide single-cause explanations. Slow productivity growth in the past 25 years echoes slow productivity growth in the late 19th century. Perhaps both were normal, and what needs to be explained is not the post-1972 slowdown, but rather the post1913 ‘‘speedup’’ that ushered in the glorious 60 years between World War I and the early 1970’s in which U.S. productivity growth was much faster than before or after. This paper makes a sharp distinction between MFP growth calculated from inputs that combine simple measures of labor hours and the capital stock and growth based on measures that adjust for the changing composition of labor and capital. The first step toward an understanding of long-term trends is to compare like with like, splicing MFP data based on unadjusted inputs prior to 1950 with post1950 data based also on unadjusted inputs, as contrasted to the composition-adjusted inputs that are now desirably incorporated into our official MFP measures. The MFP record prior to 1929 still rests largely on the monumental work of John Kendrick (1961) which, however, is based almost entirely on input quantities that lack any adjustment for changes in composition. Edward Denison ( 1962, 1985 ) and Zvi Griliches (1960) pioneered the development of composition adjustments for labor input. Dale Jorgenson and Zvi Griliches (1967) introduced a framework that treats the problem of composition adjustment in both labor and