It is useful to begin the book on the theory of chromatographic separation methods with a definition of chromatography. However, several such definitions can be formulated according to various classification aspects. For the sake of accuracy a phenomenological definition, a molecular kinetic definition and various working definitions can be introduced. According to the first definition chromatography is understood as ^phenomenon of differential migration of solute compounds in a system of two phases, of which one is stationary and the other mobile. According to the molecular kinetic definition, chromatography is taken as a continuous process of convective upsetting and diffusional reestablishment of equilibrium between the concentrations of the solute compound in the stationary and in the mobile phase of the chromatographic system. This process results in a differential migration of the solute compounds. According to the working definitions chromatography is a certain method (specifically a separation and analytical method and various methods of physicochemical measurements). From the point of view of the theory of chromatography we are particularly interested in the chromatographic process...