This chapter is a continuation of Chap. 6 in the sense that, for very complex ions, not only the number of levels is overwhelmingly large, but also the number of configurations. The concept of superconfiguration has been introduced for a proper gathering of configurations through the definition of supershells (collections of ordinary subshells). This concept relies on the notion of partial Local Thermodynamical Equilibrium, i.e., all the configurations belonging to a given superconfiguration are distributed according to a Boltzmann law at some temperature.