Many new vehicle technologies are claiming to be the best in class to reduce the impact on the environment. However what are ‘green’ or ‘clean’ vehicles? How can this be assessed in the appropriate scientific way? The underling assessment compares the environmental aspects of mainly compressed natural gas (CNG) and Battery electric vehicles, along with Liquid Petrol Gas (LPG), Biogas (BG), plug-in hybrid electric vehicles (PHEV), hybrid electric vehicles (HEV) and conventional diesel and petrol vehicles.