Thermoelectric generators have no moving parts, are silent in operation and are compact, and, when coupled to an isotropic heat source, can operate unattended over long periods in inhospitable and inaccessible environments. In recent years, thermoelectric generators have found increased application in situations where combinations of their desirable properties outweigh their relatively high cost and low generating efficiency. This paper reviews the present 'state of the art' of generating electricity by the thermoelectric effect and looks at future prospects. A brief history of thermoelectricity is followed by definitions and an introduction to its basic theory. Factors that influence the selection of materials for a particular application are discussed, together with the preparation and properties of established materials. A Section dealing with the fabrication of thermoelements and the various configurations employed in module construction is followed by a general discussion of thermoelectric generators. These are divided into two types: nonisotopic powered and isotopic powered. Emphasis is placed on the latter, and applications of this type of generator are covered in some detail. The conclusion is reached that the development and application of thermoelectric generators is assured with the continuation of the US space programme, with radioisotopic thermoelectric generators (r.t.g.s) providing onboard power for deep-space missions. In marine applications, the requirement for isotopic-powered generators in submerged situations will increase, whereas their prospects in surface and terrestrial applications is not so certain. From an economic point of view, it seems unlikely that the use of radioisotopic generators can be justified in situations where air is available to burn hydro carbon fuel. In situations where it is difficult to get fuel to the site, a thermomechanical generator provides a possible alternative to thermoelectric generators. In medical applicat