This monograph, in its third revised and enlarged edition, presents a general analysis of the distribution of crystal orientations and of dislocation density in a polycrystalline system. The information recovered from the diffraction of X-rays gives a new account of the structural states of a polycrystal and makes shear phase transformations in metals a describable process: at the macroscopic level as textural transformation, and at the microscopic level as martensitic transformation. The third edition adds a method of determining long-range order in the dislocation structure of crystals, which in practice yielded important results for understanding the kinetics of the martensitic transformation during the hardening of steel. The first part is devoted to the crystallographic texture of deformed metals: the theoretical probability distribution of crystal orientations and the texture function; the design of an optimal diffraction experiment for its harmonic analysis; the determination of the parameters of the texture function from measured harmonics; and the stability of rhombic texture and the anisotropy it creates. The second part turns to the dislocation structure of strongly distorted crystals: the theoretical foundations of its diffraction study, the estimation of the parameters of the dislocation system from measured harmonics of the diffraction line, the inhomogeneous density of dislocations in a polycrystalline system, and optimum measurements of diffraction on polycrystals for structural analysis. The methodology is embodied in problem-oriented software, supplied with the book, which automates research on deformed metals with crystals of cubic symmetry - the principal class of structural materials. Mathematical modelling, experimental design, optimal statistical estimation and the simulation of the system under study and of its evolution under loading serve as the instrumentation, and the automation programs have passed practical testing. Intended for researchers, engineers and graduate students working in the physics of metals, materials science and X-ray structural analysis.