The success of model-based approaches to systems health management depends largely on the quality of the underly- ing models. In model-based prognostics, it is especially the quality of the damage progression models, i.e., the models describing how damage evolves as the system operates, that determines the accuracy and precision of remaining useful life predictions. Several common forms of these models are gen- erally assumed in the literature, but are often not supported by physical evidence or physics-based analysis. In this paper, using a centrifugal pump as a case study, we develop differ- ent damage progression models. In simulation, we investigate how model changes influence prognostics performance. Re- sults demonstrate that, in some cases, simple damage progres- sion models are sufficient. But, in general, the results show a clear need for damage progression models that are accurate over long time horizons under varied loading conditions.