Compared with its shares in world consumption and reserves, natural gas holds a small share in international energy trade. In addition, the nature of international trade in gas is relatively fragmented. This collection of articles by leading authorities examines the reasons for these phenomena, including the substantial capital costs of gas supply systems, compared with those of oil or coal, and the technical rigidities associated with gas supply, which militate against the development of a "world gas market". The resultant difficulties of devising appropriate and enforceable long-term contracts are also discussed.