This chapter deals with Fanno and Rayleigh flows, which are the two major types of compressible flow through pipes and ducts. Section 6.1 starts off with an introduction to both Fanno and Rayleigh flows. Section 6.2 is devoted to adiabatic flow through pipes and ducts with friction loss, also known as Fanno flow. This section presents a detailed analysis of Fanno flow including the application of momentum equation to derive the length of the duct to achieve a specific change in Mach number. Topics dealt with Sect. 6.2 also include T-s diagram for Fanno flow, appropriate value of friction factor to be used, equations for temperature, pressure, density, and stagnation pressures across the pipe/duct, sonic length, star equations for Fanno flow, Fanno flow tables for air and their use in problem solving, Fanno line, Fanno flow with normal shock, and frictional chocking with maximum possible length. Section 6.3 is devoted to frictionless flow in pipes/ducts with heat transfer, also known as Rayleigh Flow. This section presents a detailed analysis of Rayleigh flow including energy balance and equations for temperature, pressure, density, stagnation pressure, and stagnation temperatures. This section also includes the topics of Rayleigh line, star equations for Rayleigh flow, Rayleigh flow tables for air, thermal chocking, and maximum possible heat transfer.