The recent development of the helium isotope refrigerator, using mixtures of the two isotopes of helium, is probably the most important step forward in the technique of producing low temperatures since adiabatic demagnetization. This article attempts to describe the special properties of3He-4He mixtures, and to show how these properties are used to produce refrigeration below 0.1 °K. Design details of actual refrigerators are given, and the direction of future improvements is suggested by a consideration of simple models of the various refrigerator components.