An effective simulation tool, CarboNitriTool© ,  to predict the carbonitriding performance of a variety of steels has been developed. This tool was needed not only to predict the hardness, carbon and nitrogen concentration profiles but also to optimize the process in terms of cycle time and cost. To develop the tool, it was necessary to predict the concentration profiles and phase development during the process. In the present work, diffusion and thermodynamic calculations and experimental work has been completed to determine the effects of process parameters on the concentration profiles and phase development in the case. The industrial carbonitriding performance for several steels was also evaluated in terms of weight gain, total fluxes, microhardness profiles, and carbon and nitrogen concentration profiles. These data were used to develop the simulation tool CarboNitriTool© based on a finite difference numerical method. In this presentation, the simulation tool will be presented and discussed in terms of database needs and process parameters.