The EPA implemented the Urban Bus Retrofit/Rebuild (UBRR) Program for transit buses built before 1994 in an effort to lower the amount of PM emissions in densely populated urban areas. The objective of the program is to provide certified emission control technologies that reduce PM emissions from older buses by 25% or to below 0.1 g/bhp-hr. This paper reviews the development of a retrofit kit that has been certified under the UBRR program to meet the 0.1 g/bhp-hr PM emission requirements on DDC 6V92TA engines with both mechanical (MUI) and electronic (DDEC) fuel injection controls. The kit is a combination of specific and modified engine parts and a catalytic exhaust after-treatment device.The kit replaces existing parts with a new camshaft, a uniquely configured cylinder kit and specified turbocharger, blower and injector. For the MUI engines the cam timing, injector height and fuel modulator are set at specific values to achieve the lowest possible PM level. For the DDEC engines, a specific engine control program and a timing offset key are used to fine-tune injection timing to minimize emission levels. An oxidation catalyst is used to provide additional PM reduction to meet the 0.1 g/bhp-hr emission level.The retrofit technology was tested under the EPA Heavy-Duty Diesel transient FTP cycle. The retrofit kit development and performance test results are discussed. Field test results from buses with retrofit engines are also reviewed.