This paper argues that analysis of seasonal fluctuations can shed light on the nature of business-cycle fluctuations. The fundamental reason is that, in many instances, identifying restrictions about seasonal fluctuations are more believable than analogous restrictions about nonseasonal fluctuations. The authors show that seasonal fluctuations provide good examples of preference shifts and synergistic equilibria. They also find evidence against production smoothing and in favor of unmeasured variation in labor and capital utilization. In some industries, capacity constraints appear to bind. Copyright 1996 by MIT Press.