Exhaust Gas Aftertreatment in Mobile Systems: Status, Challenges, and Perspectives Looking at exhaust-gas after-treatment systems in its entirety leads to further improvement of emission control devices and to the accomplishment of future challenges such as lower legislative emission limits, new fuels, and more efficient engine concepts. This article provides an overview on the state-of-the art mobile exhaust-gas after-treatment devices. Current and future challenges are discussed in the light of present approaches such as hierarchical modeling reaching from DFT computations of molecular processes to CFD simulation of entire lines of exhaust-gas cleaning devices, close-to-reality emission control test benches and aging procedures, on-board diagnostics, and catalyst characterization at operating conditions and preferentially all length scales. In future, knowledge-based robot-controlled preparation and dynamic models coupled with information from real operation will significantly support research and development.