Hybrid electric vehicles (HEVs) have been identified as a prime technology in this modern world, lowering pollution from emissions and having great potential to reduce fuel consumption. They have proved to be an effective solution for minimizing oil dependency and improving air quality. These vehicles are powered by an electric motor, power electronic equipment, and an internal combustion engine (ICE). They are a promising approach to emission-free and environmentally friendly vehicles without affecting the vehicle's functionality and its driving performance. This chapter focuses on introducing HEV systems with their historical development, electric drive technologies, operation modes, and configurations. Also discussed are the power flow modes and power train concepts of HEV systems, and the different types of HEV based on the nature of their power sources. © 2023 selection and editorial matter, Dharavath Kishan, Ramani Kannan, B Dastagiri Reddy and Prajof Prabhakaran; individual chapters, the contributors.