Technical Abstract: Determining the correct parameters for different components of an agricultural system model is a major and difficult task that currently restricts the widespread application of models in field research and technology transfer. This chapter presents current approaches for parameterization of major system components, and future research needs to improve these approaches. The approaches include example table values or equations. Two examples of how to obtain parameters for two agricultural system models are presented. The presentation of this chapter is organized into seven sections: (1) soil physical parameters; (2) soil hydrologic parameters; (3) evapotranspiration parameters; (4) soil carbon/nitrogen dynamics parameters; (5) crop growth parameters; (6) soil-pesticide parameters; and (7) statistical methods for model calibration. Finally, two examples on how to parameterize agricultural system models are presented. Other processes and parameters not covered in this chapter are runoff, soil erosion, chemical (N and P) movement, parameters related to rangeland and grazing processes, dairy models, insect damage, weed damage, and effects of natural disasters.