SINCE THE EARLY 1980s the U.S. economy has gone through two long expansions. The first, from 1982 to 1990, lasted thirty-one quarters. The second started in 1991 and, although showing signs of faltering, has recorded its fortieth quarter as this volume goes to press and is already the longest U.S. expansion on record. One view is that these two long expansions are simply the result of luck, of an absence of major adverse shocks over the last twenty years. We argue that more has been at work, namely, a large underlying decline in output volatility. Furthermore, we contend, this decline is not a recent development— the by-product of a “New Economy” or of Alan Greenspan’s talent. Rather it has been a steady decline over several decades, which started in the 1950s (or earlier, but lack of consistent data makes this difficult to establish), was interrupted in the 1970s and early 1980s, and returned to trend in the late 1980s and the 1990s. The magnitude of the decline is substantial: the standard deviation of quarterly output growth has declined by a factor of three over the period. This is more than enough to account for the increased length of expansions.