In this chapter, we will discuss the following key points: the movement of water in the reaction turbine; the basic equation and efficiency of the hydro turbine; the working principle of the draft tube; the concept of cavitation, the suction head and installation elevation of the hydro turbine; the pressure pulsation characteristics of a hydro turbine. Students should be acquainted with the basic equation of turbine and the working principle of the draft tube; understand the optimum work conditions and the influencing factors of efficiency; master the concept of cavitation and the method to determine the installation elevation of the hydro turbine; be familiar with the flow patterns and pressure pulsation characteristics of a hydro turbine.