Models for chemical evolution of the Galaxy are presented, where the most recent ideas on stellar nucleosynthesis and supernova (SN) progenitors are taken into account. It is assumed that the disk forms by accretion of primordial material coming from outside with a time-scale which is a function of the galactocentric distance. The possibility of forming the disk out of enriched halo gas is also studied in a simple way. The evolution of the abundances of several single elements is predicted as well as that of gas mass, SN rates and star formation rate, both in the solar neighborhood and in the whole disk, under different assumptions for the star formation rate. Abundance gradients along the galactic disk are well reproduced and a possible interpretation for the differences among gradients of different elements is given in terms of nucleosynthesis and stellar lifetimes. This interpretation can represent a key for understanding gradients also in external galaxies. In particular, a natural explanation is suggested for the positive gradient of S/O, found in M101 and M33.
