The paper argues that it is important to realize that the concept of a cycle has rarely been precisely articulated in empirical work and that often researchers are using very different definitions of it. We propose a two-fold classification based upon what series one is measuring a cycle in and how one would recognize a cycle in such a series. The paper illustrates how one can then categorize existing research based upon how it answers these questions. It also shows that the existence and properties of a cycle differ greatly depending upon which of the categories the researcher is using.
