This book deals with the deposition of a certain class of materials, namely the diamond-like superhard materials, in the form of thin films. Starting from concepts used to improve the hardness of a material, it is shown that the intrinsic hardness is highest for materials with low bond lengths, high coordination numbers, and low ionicities. It is further shown that such diamond-like materials (e.g. diamond, cubic boron nitride, tetrahedrically bonded carbon nitride) are distinguished by a variety of other unique and extreme properties.