This chapter discusses selected aspects of compressible flow theory which are of key importance to the study of detonation phenomena. Following classical developments on compressible flow theory, the chapter starts with the isentropic flow theory, presenting the relationships between stagnation properties and flow properties. Moreover, the different qualitative behaviors of subsonic and supersonic flows in geometries with area increase (or decrease) are discussed. Through this approach it will be possible to understand the general characteristics of steady one-dimensional compressible flow. Afterwards, the discussion is focused on the normal shock wave theory and oblique shock wave theory. The shock waves are treated as discontinuities which cause changes in the thermodynamic properties, such as temperature, pressure, and density, among others, as well as in the flow Mach number. Important aspects as the Rankine–Hugoniot relation are presented. The chapter is closed with a brief discussion on shock reflection. Several examples are provided to showcase the application of the theory.