The aftertreatment challenge in the non-road market is making the same system work and fit not just in one machine, but in hundreds of different machines, some of which can be used for many different purposes. This huge diversity of applications and the relatively small unit numbers for each application, coupled with the rapid introduction of new standards and the very high performance needed from the engines and machines, requires a sophisticated approach to product development. Furthermore, as emissions requirements become ever more stringent, designing a system to meet the legislation subject to packaging and cost constraints becomes progressively more difficult. This is further exacerbated by increasing system complexity, where more than one technology may be required to control all the legislated pollutants and/or an active control strategy is involved. Also a very high degree of component integration is required. Therefore aftertreatment system modeling is an extremely valuable tool in aiding design of systems that can meet all these challenges.Here the application of 1-dimensional numerical models, based on chemical kinetics derived for real-world catalysts, for a vanadium oxide ammonia selective catalytic reduction (SCR) de-NOX catalyst and a PGM-based diesel oxidation catalyst (DOC) are described. Both models have been developed with particular emphasis on the simulation of emissions control performance for non-road vehicles/engines over the highly transient Non-Road Transient Cycle (NRTC).These models have then been applied to investigate the sensitivity of various design parameters on the performance of the system. In particular, the effect of inlet NO₂/NOX, ammonia to NOX ratio, minimum urea injection temperature and catalyst volume on an SCR-only system and the effect of DOC volume and PGM loading on a DOC + SCR system have been investigated. These are important factors in the design of effective and flexible non-road emissions control systems.