The report is concerned with the control of NOx emissions from diesel engines. The objective was to correlate engine design, performance, and emission characteristics by theoretical analysis, and to develop NOx-control criteria. Mathematical models of combustion and NO formation in diesel engines were developed; engine design, performance, and emission characteristics were correlated; and methods for controlling NO emissions were identified and evaluated. In direct-injection engines, NO emission rate is found to vary monotonically with fuel injection time, injection rate, and over-all fuel-air ratio. Qualitative guidelines have been defined for reducing NO emissions by manipulation of these design variables while minimizing the associated penalties in engine performance. The tendency for indirect-injection engines to produce lower NO emissions than direct-injection engines has been confirmed by the model. The direct-injection engine model has been used to evaluate turbo-charging, pilot injection, and fumigation as methods for reducing NO emissions. Of the approaches considered, only fumigation was found to decrease NO emissions.