In this chapter, the general flow equation for compressible flow in a pipeline will be derived from basic principles. Having obtained the general flow equation, the way in which flow efficiency is affected by varying different gas and pipeline parameters will be examined. Different flow regimes in gas transmission systems (i.e., partially turbulent and fully turbulent flow) will be presented. Some of the widely used transmission equations and their applications, advantages, and limitations will be outlined. This will be followed by a discussion of pipes in series, pipeline looping, gas velocity, line packing, pipeline maximum operating pressure, and some pipeline codes. The impact of gas temperature on the flow efficiency and gas temperature profile (i.e., heat transfer from a buried pipeline and Joule-Thompson effects) will be discussed. Finally, some major economic aspects and considerations in the design of gas pipeline systems will be presented.