Oscar Straniero Title: The fundamental role of the metallicity in the nucleosynthesis of heavy elements Abstract: About half of the elements beyond iron are synthesized by slow neutron captures on seed nuclei (s-process ). The main component of this process is produced in low-mass AGB stars undergoing recursive thermal pulses, while the weak component is associated to the helium and carbon burning in massive stars . In any case, the number of neutrons per seed is a fundamental parameter which control the resulting nucleosynthesis. Since iron is the most important seed, this parameter basically depends on the stellar metallicity. We will discuss how the s-process changes along the metallicity gradient of the Milky Way. This is an information of primary importance to understand the galactic chemical evolution.