Abstract: Bypass systems are crucial auxiliary systems in various types of power generation units. This paper investigates and summarizes the fundamental principles, components, classifications, capacities and functions of bypass systems. Taking the cold start-up of a steam turbine in a thermal power unit as an example, the operational processes of high and low-pressure bypasses were analyzed. By comparing the advantages and disadvantages of three types of bypass structures and their practical applications, it was found that the two-stage serial bypass system is the preferred choice for most power generation units, due to its representativeness and typicality. The paper then focuses on the application of bypass systems in six types of power generation units. Through comparative analysis of their equipment configurations, structural forms, and functional characteristics, it concludes that bypass systems are technologically mature and have well-developed control functions in thermal power units and gas-steam combined cycle units, where they have been widely implemented. They are also employed in nuclear power units, which require higher design and manufacturing standards for the equipment within the bypass. However, in integrated gasification combined cycle units, the technological maturity of bypass systems is relatively low, presenting significant opportunities for improvement. In supercritical carbon dioxide combined cycle units, they perform poorly, negatively impacting operational efficiency, and have not yet been practically applied. In compressed air energy storage systems, expander bypass systems facilitate frequent start-ups and variable condition operations of the units, but both the research content and practical applications in this area are relatively limited. Finally, based on the current research shortcomings, the paper provides an outlook on future research directions for bypass systems.