This talk will give an overview of the current state of modeling emission after-treatment systems for gasoline applications. The various challenges associated with modeling Three Way Catalysts (TWCs) as well as Four Way Catalysts (FWCs) will be discussed. Supporting future emission regulations in North America, Europe and Asia requires enhanced model predictive capabilities, which poses a great challenge for developing highly accurate models. On the other hand, there is potential for greater use of modeling to design emission systems and develop controls around it. We will review combining engine combustion models with catalyst models, and conclude with a discussion about the future of emission catalyst modeling.