Global warming, air pollution, and fuel depletion have accelerated the deployment of hybrid electric vehicles (HEVs). Apart from the energy management, the configuration of hybrid powertrains plays a central role in achieving better fuel economy and enhanced drivability. This paper comparatively summarizes the configurations, modeling, and optimization techniques of HEVs. Four types of hybrid powertrain configurations available in the market, i.e., series, parallel, power-split and multi-mode, are introduced firstly, followed by their state-of-the-art and pros/cons. Among all configurations, multi-mode hybrid powertrains are observed to have the potential for utilizing the benefits of the other three types by switching the operating modes. Subsequently, the configuration generation and modeling techniques are summarized. By adopting the automated modeling method, the entire design space can be explored exhaustively, and 14 feasible configuration types are classified based on the binary tree. Finally, the research gaps and future trends of HEV configuration studies are discussed.