In this report we give a description of the membrane formation mechanism based on theoretical and experimental knowledge of phase separation phenomena in concentrated polymer solutions. We demonstrate that different types of phase separation are responsible for the build-up of the dense skin layer and the porous supporting layer in asymmetric membranes of several materials: cellulose acetate, polysulfone polyacrylonitrile and polydimethylphenylene-oxide. The formation of the porous sublayer will be explained in terms of liquid-liquid phase separation, coalescence and gelation. Special attention will be paid to the formation mechanism of the finger-like cavities in the sublayer. A possible explanation will be given on thermodynamic and kinetic grounds, for the formation of nodular structures in the skin of the membranes.