This chapter considers compressible flow through entities such as nozzles, diffusers, and convergent–divergent (Laval) nozzles, where flow area changes in the flow field. A simple background information is provided in Sect. 4.1 by applying Bernoulli’s equation to incompressible and low Mach number flows. Section 4.2 looks in detail at pressure, velocity, and area relationships in terms of Mach number and the resulting implications for both subsonic and supersonic flows including applications for converging, diverging, and Laval nozzles. This section also looks at variation of Mach number and pressure in a correctly designed Laval nozzle and explains the distinction between exit pressure, ambient pressure, and receiver (or back) pressure. Sonic flow, sonic relationships, star equations, choked flow, and sonic area are some of the topics examined in Sect. 4.3. This section forms the bulk of this chapter and includes numerous example and practice problems, which make use of the isentropic flow tables for air in their solutions. Section 4.4 looks at the impact of ambient and/or back pressure on flow patterns through a Laval nozzle and also looks at situations where there is a mismatch between nozzle exit pressure and ambient pressure resulting in underexpanded and overexpanded nozzles. This section also takes a qualitative look at situations that cause normal shocks to occur during flow through nozzles.