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In this paper we present a new chemical evolution model for the Galaxy whichassumes two main infall episodes for the formation of halo-thick disk and thindisk, respectively. We do not try to take into account explicitly the evolutionof the halo but we implicitly assume that the timescale for the formation ofthe halo was of the same order as the timescale for the formation of the thickdisk. The formation of the thin-disk is much longer than that of the thickdisk, implying that the infalling gas forming the thin-disk comes not only fromthe thick disk but mainly from the intergalactic medium. The timescale for theformation of the thin-disk is assumed to be a function of the galactocentricdistance, leading to an inside-out picture for the Galaxy building. The modeltakes into account the most up to date nucleosynthesis prescriptions and adoptsa threshold in the star formation process which naturally produces a hiatus inthe star formation rate at the end of the thick disk phase, as suggested byrecent observations. The model results are compared with an extended set ofobservational constraints. Among these constraints, the tightest one is themetallicity distribution of the G-dwarf stars for which new data are nowavailable. Our model fits very well these new data. We show that in order toreproduce most of these constraints a timescale $\le 1$ Gyr for the(halo)-thick-disk and of 8 Gyr for the thin-disk formation in the solarvicinity are required. We predict that the radial abundance gradients in theinner regions of the disk ($R< R_{\odot}$) are steeper than in the outerregions, a result confirmed by recent abundance determinations, and that theinner ones steepen in time during the Galactic lifetime.Comment: 48 pages, 20 Postscript figures, AASTex v.4.0, to be published in  Astrophysical Journa

The Chemical Evolution of the Galaxy: The Two‐Infall Model