Abstract: Bypass systems are important auxiliary systems in different types of power generation units. This study assesses and reviews the fundamental principles, components, classifications, capacities, and functions of bypass systems. Specifically, it concentrates on the steam temperature and pressure reduction system in parallel with the steam turbine. This system mainly comprises pipelines, valves, actuators, and control systems. It controls whether the steam produced by the boiler completely enters the steam turbine or bypasses certain stages to directly enter the intermediate, low-pressure stages or the condenser. It dynamically regulates the expansion ratio and output power of each part according to various working conditions. Therefore, it plays a crucial role in unit startup, shutdown, load variation, working fluid recovery, and equipment safety protection. Then, the fundamental principles and operational processes of the high- and low-pressure bypasses for the cold startup of a steam turbine in a thermal power unit are analyzed. A comparison of the advantages and disadvantages of the three types of bypass structures and their practical applications shows that the two-stage serial bypass systems are the preferred options for most power generation units due to their representativeness and typicality. This study also focuses on the applications of bypass systems in six types of power generation units. A comparative analysis of their equipment configurations, structural forms, and functional characteristics concludes that bypass systems are technologically mature and possess well-developed control functions in thermal power units and gas turbine combined cycle units, where they have been extensively implemented. They are also employed in nuclear power units, which demand higher design and manufacturing standards for the equipment within the bypasses. However, in integrated gasification combined cycle units, the technological maturity of bypass systems is relatively low, which presents great opportunities for enhancement. In supercritical carbon dioxide combined cycle units, bypass systems perform poorly, which negatively impacts operational efficiency. Thus, they have not yet been practically applied in these systems. In compressed air energy storage systems, bypass systems enable the frequent startups, shutdowns, and variable condition operations of expanders, which result in higher output power and energy density. However, throttling loss occurs in the working fluid passing through the bypasses, which leads to a decrease in system efficiency. The use of bypass systems in this field is still in the theoretical stage, and the relevant contents and practical applications are limited. Finally, this study provides an outlook on the future research directions for bypass systems based on the shortcomings of current research. As for the integrated gasification combined cycle units, the successful experience of the gas turbine combined cycle units can be drawn upon, and upgrades in terms of equipment configurations and control modes can be considered. For the supercritical carbon dioxide combined cycle units, the feasibility of using bypass systems needs to be analyzed. If it is unsuitable, then other types of control systems, such as inventory control systems, can be applied. For the compressed air energy storage systems, the latest research progress and practical application prospects of expander bypass systems should be given attention.