Since 1989, a research and development program has been ongoing to determine the viability of equipping a DDC 6V-92TA engine (the engine most commonly used in North America for transit buses) with a diesel particulate filter system. The engine is operated on a Southeastern Pennsylvania Transit Authority (SEPTA) Neoplan bus. The initial design targets included: the reduction of particulate emissions to levels below 0.1 g/bhp.hr. as well as the 1990 Clean Air Act Amendments, for carbon monoxide and hydrocarbons. a requirement for no ancillary hardware or energy sources in order to achieve regeneration. design simplicity for easy removal and inspection. good performance for normal SEPTA in-service conditions. low cost. Initially, baseline measurements were taken to determine under what conditions the filter system would have to perform. Several initial designs were evaluated with varying degrees of success. Currently, a system has been successfully evaluated on a test track typified by duty cycles found in the inner core of Philadelphia and has been put in service by SEPTA.This paper outlines the various filter systems evaluated in the program and the findings associated with them. It further outlines the final design, tests done to date, and the future path of the program.