Computer Aided Engineering (CAE) Methodologies are increasingly being applied to assist the design of SI-engine exhaust aftertreatment systems, in view of the stage III and IV emissions standards. Following this trend, the design of diesel exhaust aftertreatment systems is receiving more attention in view of the capabilities of recently developed mathematical models. The design of diesel exhaust systems must cope with three major aftertreatment categories: (i) diesel oxidation catalysts, (ii) diesel particulate filters and (iii) de-NOx catalytic converters. An integrated CAE methodology that could assist the design of all these classes of systems is described in this paper. It employs the following computational tools: (i) a computer code for the transient exhaust system heat transfer, (ii) a computer code for the transient operation of a diesel oxidation or a de-NOx catalytic converter, (iii) a database containing chemical kinetics data for a variety of oxidation and de-NOx catalyst formulations, and (iv) a computer code which models the loading and regeneration behavior of particulate filters, with or without catalytic assistance. Application of the CAE methodology is high-lighted by referring to a number of representative case studies.