A long service life of heavy-duty diesel vehicles results in a large number of older-technology trucks and buses of various types running on roads nowadays. Cleaning up exhaust gases of these older vehicles gives an opportunity to improve air quality at affordable costs. Retrofitting older buses with diesel particulate filters (DPF) is a cost-effective measure to quickly and efficiently reduce particulate matter (PM) emissions and contribute to mitigation of air pollution in urban conglomerates. In this paper, the milestones on a way to wide-scale retrofitting of heavy-duty vehicles with DPF are discussed on the example of Israel DPF retrofit program. Crucial importance of a balanced governmental approach combining regulation and economic incentives, together with collaboration of government, academia and vehicle operators, is underlined. Main results of the one-year pilot project focused on urban and intercity buses are discussed. Impact of DPF retrofitting on particulate emissions and engine performance and maintenance aspects of in-use diesel buses is analyzed. Very high particle filtration efficiency (in average, about 97%) together with relatively low fuel economy penalty (0.6–1.8%, depending on the bus type) are proved. Vital importance of careful monitoring and correct maintenance of DPF-equipped vehicles is underlined.