The focus of this chapter is on isothermal flow in pipes and pipelines. Section 7.1 starts off with an introduction and discussion on the differences between isothermal flow, adiabatic flow, and incompressible flow. Non-ideal gas behavior, generalized compressibility factor, generalized compressibility factor chart, and non-ideal specific heats are some of the topics covered in Sect. 7.2. Section 7.3 forms the bulk of this chapter and is entirely devoted to isothermal flow. Section 7.3 derives both the differential and integral forms of isothermal compressible flow equations. This section also presents equations for temperature, pressure, density, stagnation temperature, and stagnation pressure ratios across a pipeline with isothermal flow. Star equations and sonic conditions for isothermal compressible flow are also presented in Sect. 7.3, including the equation to determine pipeline length required to effect a certain amount of change in Mach number across the pipeline. Various practical problems are considered including determining pipeline length, determining downstream pressure (and hence pressure drop), and determining throughput through pipeline with extensive use of innovative, groundbreaking spreadsheets to solve problems. Section 7.3 also illustrates the importance of using appropriate friction factor values and generalized compressibility factors to obtain accurate results for pipeline flow problems. This section provides a concrete method to obtain accurate estimation of an average value of Darcy friction factor for compressible pipeline flow via the use of spreadsheets described earlier. For the benefit of readers and users of this book, all the cell formulas used in various application-oriented spreadsheets have been provided in Sect. 7.3.