State-and-transition models have received a great deal of attention since introduction of the to range management concept in 1989. Recently, sets of state-and-transition models have been produced that can be used by agency personnel and private citizens, but there is little guidance available for developing and interpreting models. Thus, we address the following questions: 1) how is information assembled to create site-specific models for entire regions, 2) how are models constructed, 3) what are the challenges to their validity, and 4) how should models be used? We review the general structure of state-and-transition models, emphasizing the distinction between the easily-reversible changes among communities within states and the more persistent, threshold behavior of changes among states (transitions). Both succession and transitions occur, so these models are complementary. Ecological sites serve as a framework for developing and selecting models. We illustrate the importance of clearly delineating ecological sites to produce models and describe how we have dealt with poorly-delineated sites. Producing specific models requires an understanding of ecological mechanisms underlying transitions. Mechanisms tend to fall within discrete categories based upon fundamental ecological processes and their relationships are readily understood. A knowledge of mechanisms is related to the use of ecological indicators to anticipate thresholds. We conclude models should include 1) reference values for quantitative indicators, 2) lists of key indicators and descriptions of changes suggesting an approach to a threshold, and 3) a rigorous documentation of the theory and assumptions (and their alternatives) underlying the structure of each model.