The purpose of this chapter is to provide the reader with an introduction to one of the most important, but neglected, subjects in energy economics: natural gas pipelines. Many of the concepts also apply to pipelines carrying crude oil or oil products. For pedagogical reasons it is necessary to include a considerable amount of mathematical material in this chapter, but the general reader will find that the discussion presented in the first two sections is only slightly technical, and covers both pipelines and depletion of gas fields. More general surveys of the natural gas markets can be found in an article by David Hawdon (1985), in which pipelines are referred to in an appendix, in A.R. Khan (1984), K. Roland (1984) and O. Fugelberg (1984).