In this chapter, the underground powerhouse is introduced. Hydropower stations are often located in hard areas such as the alpine and gorge regions, and the underground powerhouse has become an important type of powerhouses. Therefore, in this chapter the development of the underground powerhouse is introduced firstly, including the application scenarios, as well as the characteristics of layout and construction. Next, the layout form of the underground powerhouse is analyzed, including its composition, layout principles, layout types and methods. In addition, because of the complex characteristics of underground powerhouse cavern excavation, this chapter focuses on the layout of the main cavern structure of underground powerhouse, including main and auxiliary powerhouse, main transformer tunnel and switch station, assembly site, surge chamber, draft tube and tailrace tunnel, auxiliary cavern and so on. On this basis, the layout design of the underground powerhouse is introduced from the aspects of the powerhouse layout principle and the determination of structural size. Finally, the calculation methods of structure design for crane girder on rock wall in underground powerhouse structure and the surrounding rock stability and support analysis method of underground powerhouse cavern are introduced in detail.