:Shorter new vehicle development time require new engineering methods in the concept and design phase. Therefore methods, which allow an early screening of design alternatives, have been developed and integrated in the design processes. This includes experimental (mostly bench tests) as well as simulation tools.Similar trends can be recognized in the area of exhaust gas aftertreatment. Ever tightening emissions regulations make it necessary in early stages to determine the performance of different aftertreatment concepts. Due to the fact that over 90% of the unconverted emissions occur before the converter is at operating temperature, the heat-up and light-off behavior of the converter is of major importance.In this context a numerical model has been developed to simulate the heat-up and light-off behavior. The model describes the heat transfer processes in the upstream exhaust piping and the monolithic catalyst carrier structure as well as the mass transfer and chemical reaction in the heterogeneous catalyst system of the converter. The model is validated by experiments on an engine dynamometer.On the engine test bench the heat-up and light-off behavior for different converter configurations (cell density and wall thickness of the monolith) have been studied and compared to the numerical results. Differences between model and experiments have been determined for the conversion behavior, mostly due to the problems to evaluate the appropriate catalyst reaction kinetics.In the final step the mathematical models are used to perform parameter studies. Hereby the impact of the converter layout (diameter and volume) and finally the properties of the substrate (cell density, wall thickness and porosity) have been investigated.