  In this work we analyse the role of low- and intermediate-mass stars in
contributing to the chemical enrichment of the interstellar medium. First we
present new sets of stellar yields basing on the results of updated
evolutionary calculations, which extend from the ZAMS up to the end of the AGB
phase (Girardi et al. 2000; Marigo et al. 1999a). These new yields, that
present a significant dependence on metallicity, are then compared to those of
other available sets (Renzini & Voli 1981; van de Hoek & Groenewegen 1997). The
resulting differences are explained in terms of different model assumptions --
i.e. treatment of convective boundaries, mass loss, dredge-up, hot-bottom
burning --, and further discussed on the basis of important empirical
constraints which should be reproduced by theory -- i.e. the initial-final mass
relation, white dwarf mass distribution, carbon star luminosity function, and
chemical abundances of planetary nebulae. We show that present models are able
to reproduce such constraints in a satisfactory way.
