We present the method for interpretation of adsorption in porous media called here the method of reference surfaces (MRS), which considers the volume adsorption as an evolution of a two-dimensional condensation (2DC) on the pore walls. The pore condensation equation (CE), which establishes the correlation between equilibrium pressures of monolayer adsorption on the surface under discussion and on the reference surface, is derived from the statistical mechanics and general thermodynamics. In particular, the CE is valid for evaluation of critical pressures of 2DC, p(c), on the pore surface and used for calculating p(c) for the following pairs of carbon surfaces: (1) a surface inside the slit and the reference surface outside the slit; (2) a surface of a slab made up of finite numbers of planes and the reference slab surface of infinite numbers of planes; (3) a finite surface in the XY plane and the reference unbounded surface; and (4) a cylindrical surface and the reference flat surface. For micropores, the MRS predicts filling pressures that are in good agreement with those of the DFT and molecular simulations; for cylindrical mesopores, the MRS and the Kelvin equation are compared favorably in the region of applicability of the Kelvin equation. The method is convenient for studying the edge effects and condensation on nanoparticles and applicable to adsorption of complex molecules. The MRS leads to unbiased parameters of porous structure, which allow predicting adsorption equilibriums; it provides the rational foundation for the Dubinin equations and bridges a gap between theories of adsorption on the surface and into the volume. Although the MRS conforms for describing many adsorption phenomena, there is an urgent need for the independent verification of the method.