Stringent North American emission regulations present several challenges for lean-burn diesel vehicles that offer significant fuel economy advantages over gasoline vehicles. This article presents a useful systems approach to developing robust models and combining them to analyze diesel aftertreatment (AT) technologies: (1) the diesel oxidation catalyst (DOC) that oxidizes carbon monoxide (CO), unburned hydrocarbons (HCs), and nitric oxide (NO), and stores HCs and (2) the urea-based selective catalytic reduction (SCR) catalyst that hydrolyzes aqueous urea to ammonia (NH3), which, in turn, reduces nitrogen oxides (NOx). The DOC and SCR models integrate information from multiple sourcesdetailed thermal balances, laboratory performance data, phenomenological descriptions of adsorption and desorption in the catalyst, and experience-based correlationsusing optimization and statistical tools. The DOC model predicts cumulative HC and CO tailpipe vehicle emissions as well as DOC NOx outlet composition (NO vs NO2). The SCR model uses the exotherm and NO2 information from the DOC model to predict NOx conversion and NH3 slip. System-level analyses that have resulted in important insights are highlighted using case studies, including (1) extrapolation of the downstream injection exotherm vehicle data for a fresh DOC to an aged DOC; (2) analysis of the effects of NO2 produced in the DOC, SCR aging conditions, and NH3 storage capacity on the NOx reduction performance of the SCR on a 6000-lb light-duty truck, and (3) development of urea injection strategies based on a tradeoff between NOx reduction and NH3 slip.