This paper present new evolutionary computations extending toward the lower main sequence previous computations given for globular cluster stars. We discuss the physical inputs with particular regard to the limit of the adopted usual treatment of the stellar atmosphere, which allows a tight correlation with actual stellar structures only for selected range of masses, depending on the assumed metallicity. Theoretical evolutionary properties of Very Low Mass (VLM) stars are presented and discussed. We find that adopting new extended tabulations of radiative opacity, as computed according to Alexander & Fergusson (1994) procedures and by relying on the Saumon & Chabrier (1992) equation of state (EOS) our results allow a reasonable fitting of HST observation of metal poor VLM objects in the galactic globular NGC 6397. The investigation has been extended to a sample of field VLM star with known parallaxes, showing again a satisfactory agreement with the distribution of metal poor dwarfs and discussing the present theoretical scenario for solar metallicity VLM. Selected evolutionary quantities are presented in suitable tables for metal poor objects (Y=0.23, 2=0.0001, 0.0003, 0.0006 and 0.001). A brief discussion including a comparison of mass-luminosity relations for VLM objects for the various investigated metallicities closes the paper. © 2004 Elsevier Science B.V., Amsterdam. All rights reserved.