The most outstanding and useful application of the reversible thermal and electrical effects, which occur at the junctions of certain semiconductors, is the application of the Peltier effect to small-scale cooling. The theory of Peltier cooling is outlined, together with methods of producing couples, or multi-couple units, from the basicp- andn-type thermoelements available commercially. Simple measurements can be made during construction to ensure that units perform in a predictable manner. To illustrate the particular problems involved in thermoelectric measurements, an account is given of rapid-check methods and of the more sophisticated techniques used to ascertain the quality of thermoelements and couples. The design of a manually-variable power supply capable of producing smooth direct current for a range of composite cooling units is considered, with a view to obtaining a compact reliable unit by using transductor control. A constant-temperature enclosure is described which is intended for use as a cold-junction reference for conventional thermocouples in place of the more usual ice/water reference used for temperature measurements. This is an example of a simple on-off control of cooling power. Finally the design and performance of a dew-point hygrometer are given in order to illustrate the potentialities of thermoelectric cooling applied under proportional servo-control conditions.