In this paper two models of the Friciton Stir Welding process will be discussed. The first is a thermal model which is used to simulate temperature profiles in friction stir welds. The required inputs for this model are total input power, tool geometry, thermo-physical properties of the material being welded (density, conductivity, specific heat) welding speed and boundary conditions. The output from the model can be used to rationalize observed hardness and microstructure distributions. The second model is a fully coupled, 2-D fluid mechanics based process model that is used to make parametric studies of variations in properties of the material to be welded (mechanical and thermophysical) and variations in welding parameters. Results from this model provide insight regarding the effect of material properties on friction stir weldability and on potential mechanisms of defect formation. Both models are readily implemented on a Windows or similar computing platform. Limitations and critical issues for extension of the models will be discussed.