The monograph considers numerous carbon structures, both real and hypothetical, the possibility of the formation of which is not excluded in principle. In addition to the structures of diamond and graphite, such structural forms of carbon as carbon black, onions, graphene, nanotubes, fullerenes, glassy carbon and ultradispersed diamond are described in detail. Among the hypothetical structures, much attention is paid to structures, that are built only on sp2 hybridized bonds: Н-6, bct-4 and others. The hypothetical structures of carbyne and carbinoids, which contain sp1 hybridized bonds, are considered also. The large volume of material in the book is devoted to the analysis of the regularities of mutual transformations of carbon phases at high pressures and temperatures. The influence of structural defects on the mechanisms and kinetics of phase transformations is described. Particular attention is paid to the study of phase transformations of carbon structures under high-temperature shock compression. The monograph also discusses the features of crystal chemistry and phase transformations in boron nitride, which is a crystallographic analogue of carbon. A number of sections are devoted to the features of X-ray diffraction analysis of carbon structures. The book is intended for – materials scientists, physicists, as well as for university teachers, graduate students and students of relevant specialties.