Quantifying Hybrid Electric Vehicle (HEV) emissions and fuel consumption is a difficult problem for a number of different reasons: 1) HEVs can be configured in significantly different ways (e.g., series or parallel); 2) the Auxiliary Power Unit (APU) can consist of a wide variety of engines, fuel types, and sizes; and 3) the APU can be operated very differently depending on the energy management system strategy and the type of driving that is performed (e.g., city vs. highway driving).With the future increase of HEV penetration in the vehicle fleet, there is an important need for government agencies and manufacturers to determine HEV emissions and fuel consumption. In this paper, several critical issues associated with HEV emissions and fuel consumption are identified and analyzed, using a sophisticated set of HEV and emission simulation modeling tools. Two different types of APUs are modeled, one based on a conventional gasoline Internal Combustion Engine (ICE), the other based on a small hydrogen-fueled ICE. Different energy management strategies and HEV configurations are examined, including a parallel range-extender charge-depleting HEV, a series thermostatic charge-sustaining hydrogen HEV truck, and a power-splitting charge-sustaining HEV (modeled after the Toyota Prius). Results show that HEV emissions and energy consumption have a high degree of dependency on: 1) the energy management strategy employed; 2) the length of the drive cycle; 3) overall driving range; and 4) the initial battery state-of-charge (SOC). The simulation results present: 1) equivalent fuel economy; 2) emissions per mile; 3) pure electric range; and 4) total driving range, for the different cases analyzed. The simulation modeling tools are extremely useful for comparing different HEV configurations and should play an important role in developing a robust HEV emissions and fuel consumption test procedure.