This project is one component of a broader effort whose ultimate goal is to provide CFD-based tools that can be used to optimize the design of urea SCR NOx aftertreatment systems for heavy-duty diesel engines. Here the focus is on predicting the distributions of key chemical species (ammonia, in particular) at the inlet to the catalysts. Two aspects of the physical models have been emphasized: the multi-phase models, and the gas-phase chemistry models. A hierarchy of four simplified geometric configurations has been used for model development and parametric studies, and to establish the appropriate level of physical modeling and numerical fidelity required. The resulting physical and numerical parameters then have been used to model a production SCR system. Initial quantitative comparisons with experimental measurements are encouraging. It is expected that CFD with the level of modeling that has been employed here can provide useful guidance for developing SCR systems that provide good spatial uniformity of ammonia entering the catalysts, while imposing low parasitic pressure drops. Specific recommendations are offered for effective use of CFD for this application, and issues that require further investigation are identified.