The internal combustion engine vehicle (ICEV) loses a large amount of energy in heat in the engine and idling. The electric vehicle (EV) and hybrid EV (HEV) are more efficient vehicles in the market. The EV has the highest energy efficiency among the EV, the HEV and the ICE. The EV allows diversification of energy resources; enables load equalization of power systems; delivers zero local and minimal global exhaust emissions and operates quietly. However, the commercialization of EVs is hindered by short driving range and high upfront cost. These barriers cannot be easily solved by the available EV energy source technologies, including batteries, fuel cells, capacitors and flywheels. Hybridization of the highenergy energy source and the high-power energy source in the electrical drivetrain of EVs is a viable approach to prolong driving range of EVs. On the other hand, the HEV is designed to hybridize the ICE drivetrain and electrical drivetrain to boost up efficiency of the ICE system. Hybridization of energy sources in EVs and hybridization of powertrains in HEVs can significantly boost system efficiency and driving ranges of the EV and HEV. The onboard energy source is the most important part in drivetrain hybridization in the HEV and energy source hybridization in the EV. This work discusses the energy sources for the EV and HEV applications. The drivetrain topologies of HEVs will be elaborated with the emphasis on functionality of the electrical powertrain. The topologies of hybrid energy systems (HESs) in battery EVs (BEVs) and fuel cell EVs (FCEVs) will also be discussed.