Like all sciences, ecology is characterized by periodic dramatic changes in concepts. Progressive managers will want to be apprised of these paradigm shifts. For example, the Clementsian model of vegetation dynamics (Clements 1916; Dyksterhuis 1949) still serves as the basis for the classification and management of most public lands, despite the fact that the more appropriate state-and-transition model (Westoby et al. 1989)was adopted by ecologists over a decade ago. The delay in adopting the state-and-transition model by land managers probably stems, at least in part, from the absence of an analytical technique to quantify state conditions and transition probabilities ( Joyce 1992). The state-and-transition model is described in Chapter 4.