This chapter synthesizes the different kinds of simulation models commonly used to address questions about controls on dynamics of perennial grasslands. Five major classes of models are described using specific models as examples: (1) demographic, (2) physiological, (3) physical, (4) biogeochemistry, and (5) dynamic global vegetation. Each class of model is described relative to the types of questions that can be addressed, and then its advantages and limitations are stated. Recommendations on which model to select for different kinds of questions are given as well as future challenges in grassland modeling are described. The chapter contains five tables, one for each model class, that provide detailed information on input parameters, spatial and temporal resolution, and response variables for specific models that will aide readers in selecting models for use in different situations. In addition, a comprehensive literature cited includes many of the more commonly used and published models for grasslands.