This document discusses hybrid electric vehicles (HEVs). It defines an HEV as a vehicle with two or more energy sources combined with energy storage. The objectives of HEVs are to maximize fuel economy, minimize emissions and costs, and maintain performance. Advantages over conventional engines include regenerative braking and increased efficiency. Motor components include electric motors/controllers, batteries, ultracapacitors, and a transmission. Vehicle propulsion can be in series, parallel, or dual-mode configurations. Examples given are the Toyota Prius and Honda Insight. Challenges for HEVs include improving energy storage and reducing size/costs of components.