The volume Rare Earth Elements C 11 deals with the compounds and systems of the rare earth elements with boron, i.e. borides, borates, and associated alkali double compounds. As in all earlier volumes of "Rare Earth Elements" Series C (Seltenerdelemente Reihe C) comparative data are presented in sections preceding treatment of the individual compounds and systems. Topics of the present volume C 11 a are the comparative data on the borides and the individual sections on the systems and borides containing Sc, Y, and La. The individual sections for the systems and borides with Ce to Lu can be found in the following volume C 11 b together with the borates, its alkali double compounds and other compounds containing Ce to Lu, boron and elements related by the Gmelin system. The most extensively studied borides treated in the comparative sections are of the type MB6. These rare earth hexaborides are refractory compounds and some of them, especially LaB6, are good thermionic emissive materials or exhibit other interesting physical properties. The most comprehensive chapter in the individual sections of volume C 11 a deals with LaB6, which is a well-known thermionic material of great practical importance with high electron emission efficiency and an excellent stability in the surface composition at high temperatures. The effects of preparation conditions, composition, temperature, surface structures, surface treatment, and of the atmosphere on the thermionic emission are studied thoroughly. Another important item of the LaB6 chapter is the electronic structure, which often serves as an example for the other rare earth hexaborides.