Recent regulatory attention has focussed on reducing the emissions from new heavy-duty diesel vehicles. Stringent new NOx and particulate emissions standards will go into effect in 1988 and 1991. The long lifetimes of heavy-duty trucks and buses, the high levels of emissions per vehicle, and the acute particulate problems in many cities all argue that retrofitting emission controls to existing vehicles should be considered as well. This paper presents a preliminary evaluation of eight potential diesel retrofit technologies (considered as any technique for reducing emissions from existing vehicles). Technologies considered were (in order of their apparent promise): diesel fuel modifications, trap-oxidizers, conversion to methanol fuel, particle agglomerators, fumigation with LPG, vertical exhausts, special-purpose fuel additives, and high-altitude adjustments/kits. The technical feasibility, emissions benefits, approximate cost, and cost-effectiveness of each technique are discussed.