The noteworthy and recent increase in interest about GaN originates from the fact that it is a very promising material for applications in high-power and high-frequency electronic and optoelectronic devices. Accordingly, this volume deals primarily with the preparation and properties of this important III-V compound. GaN is a n-type semiconductor but it is possible to also get p-type GaN by doping. Many methods of growing monocrystalline epitaxial films, especially on sapphire, Al2O3, have been developed and are described in detail in the volume. Additional advantages of GaN are its stability at high temperatures and its resistance to most chemicals.