In local universe, passive galaxies represent only ~20% of the total number of galaxies, but contribute for more than 60% to the total mass. As a consequence, the comprehension of the mechanisms driving their stellar mass accretion is a fundamental step to understand the universe we see today. In the last five years, deep HST surveys have given a boost to our knowledge on massive quiescent galaxies, erasing some doubts and opening new ones. In this talk I present some of the recent results, both on local and high-z (z ~ 2) passive galaxies, which allow us to gain insight on and to constrain their stellar mass assembly history.