The worldwide trends for future CO2 regulation standards will push car manufacturers for more and more development of Electric and Hybrid Electric Vehicles. Many different configurations of Hybrid Electric Vehicles exist, including parallel hybrid, series hybrid, plug-in hybrids, Battery Electric Vehicles with Range Extender, etc. The choice of the optimal architecture depends on many different parameters, and is a key issue to be solved at the beginning of vehicle development.In order to help decision making in the early phase of projects, simulation tools are essential. A specific simulation platform for simulation of fuel economy and CO2 emissions for hybrid electric vehicles has been developed by Renault. This platform called GREEN is dedicated to the selection of vehicle and powertrain architecture, to the pre-design of powertrain components, and to the determination of Fuel Economy and CO2 targets in the early phase of projects, and to monitoring of Fuel Economy and CO2 during vehicle development.