The paper presents the analysis of general approaches for high-vacuum pumping systems choice depending on service conditions, which is essential to provide dry vacuum system pumping that is determined by various technological processes. It describes the main structures of modern combined turbomolecular vacuum pumps. Generally molecular flow parts as forevacuum stages are mounted in the researched combined turbomolecular vacuum pumps. The analysis of prospective developments in the researched pumps has been carried out as well. Molecular - viscous flow parts that have a number of advantages in comparison with molecular flow parts are considered as an alternative to molecular stages. A gas flow in molecular stage channels of a combined turbomolecular vacuum pump in a viscous flow mode has been investigated by means of STAR-CCM+ fluid dynamics software package. Design parameter are provided for the whole turbomolecular vacuum pump flow part taking into account the effect of running clearances between a pump stator and its rotor.