  In this paper we present a new chemical evolution model for the Galaxy which
assumes two main infall episodes for the formation of halo-thick disk and thin
disk, respectively. We do not try to take into account explicitly the evolution
of the halo but we implicitly assume that the timescale for the formation of
the halo was of the same order as the timescale for the formation of the thick
disk. The formation of the thin-disk is much longer than that of the thick
disk, implying that the infalling gas forming the thin-disk comes not only from
the thick disk but mainly from the intergalactic medium. The timescale for the
formation of the thin-disk is assumed to be a function of the galactocentric
distance, leading to an inside-out picture for the Galaxy building. The model
takes into account the most up to date nucleosynthesis prescriptions and adopts
a threshold in the star formation process which naturally produces a hiatus in
the star formation rate at the end of the thick disk phase, as suggested by
recent observations. The model results are compared with an extended set of
observational constraints. Among these constraints, the tightest one is the
metallicity distribution of the G-dwarf stars for which new data are now
available. Our model fits very well these new data. We show that in order to
reproduce 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 solar
vicinity are required. We predict that the radial abundance gradients in the
inner regions of the disk ($R< R_{\odot}$) are steeper than in the outer
regions, a result confirmed by recent abundance determinations, and that the
inner ones steepen in time during the Galactic lifetime.
