▪ Abstract  In the last decade of the twentieth century there has been a significant increase in research on a more than 100-year old phenomenon—the magnetocaloric effect (MCE). As a result, many new materials with large MCEs (and many with lesser values) have been discovered, and a much better understanding of this magneto-thermal property has resulted. In this review we briefly discuss the principles of magnetic cooling (and heating); the measurement of the magnetocaloric properties by direct and indirect techniques; the special problems that can arise; and the MCE properties of the 4f lanthanide metals, their intra-lanthanide alloys and their compounds [including the giant MCE Gd5(SixGe1−x)4 phases]; the 3d transition metals, their alloys and compounds; and mixed lanthanide-3d transition metal materials (including the La manganites).