The impact dynamics of a droplet onto a solid surface are important in a variety of applications, such as inkjet printing and spray coating. Many fluids encountered in practical industrial applications exhibit non-Newtonian behavior, and therefore more research associated with non-Newtonian fluids is necessary. This paper reports on a numerical study of the impact dynamics of yield-stress fluid droplets. The numerical simulation is performed using a computational fluid dynamics package, Fluent 6.3, with a volume of fluid model. The numerical simulation models the presence of yield-stress and shear-rate dependent viscosity using the Herschel-Bulkley rheological model. The numerical results are found to be in qualitative agreement with experimental data in the literature. By performing extensive numerical simulations varying the impact velocity, rheological parameters, and surface tension, the influence of these parameters on the impact dynamics are evaluated, and the dominant effects that govern the spreading and relaxation phases are determined. The results show that while the spreading behavior is determined by the power-law index n, the non-Newtonian Reynolds number Re-n, and the Weber number We, the retraction behavior is determined by the non-Newtonian capillary Ca-n and the Bingham-capillary number (B) over cap. In addition, the scaling law that predicts the maximum spreading diameter is presented. (C) 2012 Elsevier B.V. All rights reserved.