Computer modeling is a valuable tool for the design and optimization of aftertreatment systems. Here the model development process adopted at Johnson Matthey will be presented. In the area of diesel emissions especially, there are many catalysts and combinations of catalysts available to aftertreatment system designers, e.g. NOx traps, selective catalytic reduction (SCR), diesel oxidation catalysts (DOC), and particulate filters; by using computer modeling it is possible to greatly accelerate the system design process. Furthermore, with ever tightening emissions legislation, optimization of these potentially complex systems is becoming increasingly important. For example, in a DOC plus SCR system, the NOx reduction over the SCR catalyst can be significantly enhanced by optimizing the ratio of NO/NO₂ produced by the upstream DOC. Furthermore, upstream NO₂ formation is a key process in the successful operation of a passive particulate filter system. Examples of DOC modeling to simulate NO₂ production by a DOC in a diesel exhaust system will be presented. Extending the use of modeling, it is also possible to simulate emissions over the full engine map; this can be an important aid to engine calibration engineers, and will be shown here.