To elucidate the formation mechanisms of polymeric asymmetric porous membranes by wet phase separation we postulated a set of elementary processes (EPs) so that a combination of some of them in time and in space forms a particular membrane formation mechanism. Individual EP ought to be recognisable through some particular experimental fact and/or through their combination. Turbidity phenomena taking place during the transformation of the cast solution into protomembrane, shrinkage of the membrane-forming ternary system during the (proto)membrane formation, pure water permeability of the formed membrane and the membranes' cross section morphology, obtained by scanning electron microscopy are, beside some phenomena occurring by the double layered membrane formation, the experimental methods used for the recognition of individual EPs and consequently for the mechanisms of the polymeric membranes formation by wet phase separation. EPs are presented in some detail and used in the presentation of the formation of particular parts of the polymeric membrane (skin, body, macrovoids) as well as for the presentation of mechanisms of the polymeric asymmetric porous membranes formation from particular polymer/solvent/nonsolvent ternary systems (polymers: cellulose acetate, polysulfone, polymethylmetacrylate, polyamide, polyurethane; solvents: acetone, N,N-dimethylformamide and acetamide; nonsolvent: water).