A t …rst glance, many macroeconomic time series exhibit some form of nonlinearity. For instance, output growth and in ‡a-tion show less volatility in the 1980s and 1990s than during the Great In ‡ation period of the 1970s, an observation that has been labeled the Great Moderation. Over the business cycle, the unemployment rate exhibits an asymmetric sawtooth pattern whereby it rises rapidly during downturns and declines only gradually during a recovery. Many price variables, such as exchange rates or commodity prices, appear stable for a long period followed by sudden level shifts. The literature has studied various speci…c forms of nonlinearity-such as structural breaks, time-varying volatility, or business cycle asymmetriesusing sophisticated time-series methods ranging from threshold and Markov switching to vector-autoregressions (VARs) with time-varying parameters and stochastic volatility. The result as to whether there is nonlinearity in the data has been mixed. 1 A key issue in this