In this chapter, we discuss the propagation of a small disturbance through a fluid. First, the entire flow, including the flow induced by the disturbance, is assumed to be a potential flow (irrotational flow) as in the usual textbook, and it is shown that the disturbances of velocity, density, and pressure are all governed by the wave equation. Next, the case where the flow is not irrotational is discussed, and it is shown that the disturbances of density and pressure are governed by the wave equation but the disturbance of velocity is not. After that, we discuss planar and spherical sound waves. Finally, we touch the frozen sound speed and the equilibrium sound speed.