Book excerpt: In the present work, in Part I, new elements widening the bases of the defects theory are shown; particularly, this work discusses the mechanism of the formation of defects as a result of elementary processes; the work also presents a description of the equilibrium state by one equation, taking into account the concentrations of defects and their changes during the process of reaching the equilibrium. The equation relates the concentrations of defects, dependent on the standard Gibbs energies of their formation, with the equilibrium pressure of oxygen. The obtained relations and the discussion have lead to the development of a method for determination of complete diagrams of concentrations of the point defects, which allows taking into account the minority defects. Part II presents the results of the calculations of the diagrams of point defects’ concentrations for a series of pure and doped oxides of transition metals 3d, with different composition (M/O ratio), crystallographic structure and point defects’ structure. A critical analysis of the results of the studies of the deviation from the stoichiometry and the electrical conductivity, obtained (so far) by most research groups has been also performed. A new interpretation of these results, concerning concentrations and types of ionic defects present has been performed and the mobility of electronic defects and its dependence on the temperature and oxygen pressure have been determined.