On the basis of analysis of adsorption and capillary phenomena occurring in adsorbent pores, a rational classification of pore varieties is given (macro-, transitional and micropores) and, as a consequence of its development, a classification of porous adsorbents and catalysts into basic structural types (macroporous, transitional-pore, and microporous). For the macro- and transitional-pore types of adsorbents, the basic parameters of the porous structures are: the pore volume, the specific surface area and the pore volume distribution according to the effective radii, corresponding to an equivalent model porous body with an assumed geometrical pore shape. The conventional nature of the distribution curves as characteristics of the porous structure is shown. For microporous adsorbents, in accordance with the theory of micropore filling, the basic parameters of the porous structure are: the micropore volume and the constant of the distribution function, B, expressed by the characteristic adsorption equation. The value of the constant B depends on the micropore sizes predominant in the structure. Examples of the presence of two microporous structures in a number of actual microporous adsorbents are quoted. An analysis of the specific features of adsorbents of mixed structural types is given, and rational methodical ways for determining the parameters of the porous structure for all the pore varieties in actual adsorbents are formulated.