In the search of improved EOS the results of molecular theory and computer simulations are extensively used, special attention being paid to effects of molecular size, polarity and the ability of molecules to associate. The relation of various EOS to underlying molecular models is discussed, the advantages of the SAFT EOS as the most theoretically sound equation are emphasized. New results are presented concerning the application and modification of the hole group contribution model proposed in our previous works. This model has proved to be universal and sufficiently elaborated for phase equilibria calculations over a wide range of temperatures and pressures for pure and mixed fluids of different chemical nature. However a simultaneous accurate description of single phase regions and of saturation curves using this model or other classical EOS is a problem.