A computer model named YawDyn has been developed at the University of Utah for the analysis of yaw dynamics of horizontal axis wind turbines. This paper reports results of a validation study of YawDyn using full-scale test results from the Combined Experiment rotor developed and operated by the Solar Energy Research Institute (SERI). A primary finding of recent research is that stall hysteresis has a large influence on yaw moments on the Combined Experiment rotor. Instantaneous pressure distributions measured on the rotor provide conclusive evidence that stall hysteresis is present and calculations using YawDyn show that the hysteresis increases the mean yaw moment dramatically (typically over 100 percent) and the cyclic yaw moments to a lesser extent. This paper provides details of the effect of stall hysteresis and shows the results of representing hysteresis using a Gormont model for dynamic stall. Results of validation efforts using YawDyn to analyze both fixed and free-yaw behavior demonstrate the model is qualitatively correct. The quantitative accuracy of the model varies depending upon the test conditions and the rotor. In some situations the model is adequate while in other situations the accuracy must be improved. Consideration of stall hysteresis and skewed wake aerodynamics is vital to the success that has been obtained to date.