The main properties of the rare earth elements are discussed, emphasizing their magnetic properties. However, it is difficult to use these elements in practice, because their Curie points are located below room temperature. In order to take advantage of the properties of these substances, they can be alloyed with iron, cobalt, or nickel. The properties of the rare earth-cobalt alloys are discussed, the phase diagram and the crystal structures are described, and the ferrimagnetic behavior is explained. Alloys of interest for magnet applications are selected.