The problem of pore formation in unlimited (very large) volume has been considered. General equations describing the system have been obtained for three possible situations of pore formation: Monte Carlo-like case, entropy mechanism of pore formation, and thermodynamically limited situation. The applicability of each approach to real systems has been considered. It has been proved that, in the fractal description of pore formation, the geometry of a porous cluster-random fractal does not differ from that of the initial fractal. The values of surface area and fractal dimensionality in various situations have been estimated. Solutions following thermodynamic limitations have been compared to those obtained for random (Monte Carlo approach) situations.