Focuses on an important aspect of this highly diversified area of condensed state physics: the one-body approximation in the theory of disordered systems. It describes the scope of problems within the framework of this approximation, its use in formulating several basic concepts, and its value in revealing many characteristic features of disordered systems. The book's main focus is on the density of states and the space-time correlation functions, and on their basic thermodynamic and kinetic characteristics. Among the many areas explored are the general properties of the one-body models frequently used and descriptions of selected one-dimensional problems, including closed dynamical equations; these are then used to thoroughly explore the density of states for several systems. In addition, some of the more complex characteristics of one-dimensional disordered systems are examined using the Fokkerr-Planck equations developed earlier in the text. Also includes a description of the general structure of concentration expansions, giving examples of simple applications.