The United States Environmental Protection Agency's (EPA) National Center for Advanced Technology (NCAT), located at its National Vehicle and Fuel Emissions Laboratory in Ann Arbor, Michigan, has been a global leader in development and demonstration of low-greenhouse gas emitting, highly fuel efficient series hydraulic hybrid drivetrain technologies. Advances in these exciting new technologies have stimulated industry to begin manufacturing hydraulic hybrids for both commercial truck and non-road equipment markets. Development activities are continuing for other markets, including light-duty vehicles. Given the commercial emergence of these low-greenhouse gas emitting series hydraulic hybrids, EPA has passed the leadership for further development to industry.This paper focuses specifically on the design and demonstration of the final series hydraulic hybrid drive for commercial trucks and buses as designed, built and demonstrated by EPA, incorporating many of innovative hydraulic technologies designed and advanced throughout EPA's hydraulic hybrid program. The unique integrated hydraulic drive module and its transmission in a series hydraulic hybrid vehicle (HHV) configuration were installed in a 30-foot class 6 diesel midibus. This HHV was able to achieve 50-180% better mpg over various urban stop-go drive cycles and 30% better mpg over the EPA highway cycle in chassis dynamometer testing. The paper shows design, control and integration techniques necessary for a cost-effective series hybrid for the commercial truck and bus market. Benefits include significant improvements in hydraulic pump/motor efficiency, maximum fuel economy for stop-go cycles while also improving mpg on highway operation, simple installation and removal of system components, and incorporation of safety features to address on-highway application of hydraulic hybrid technology.