Technologies for emission reduction from diesel (a.k.a. compression ignition) engines can be categorized as pre-combustion elements, in-cylinder combustion elements and post-combustion elements. Many technologies reduce emissions, yet have difficulty in simultaneously improving engine performance, saving fuel or reducing costs, all four of which are critical elements to successful technology introduction. Most of these technologies are being developed as discrete systems and often face frustrating “trade-offs” such as emission reductions causing engine performance degradation and/or fuel penalties, or the common PM/NOx dilemma. This paper outlines the key trade-offs of the four criteria, discusses various technologies and their limitations, and suggests a systems-level optimization strategy for a cleaner diesel engine.