All modern determinations of chemical abundances of He/H, O/H, C/H, N/H and Ne/H in 277 galactic Planetary Nebulae have been used to update our knowledge of the chemical behavior of Planetary Nebulae, as a whole and in Peimbert's groups. The data have been used to set up the problem of the chemical gradients in the Galaxy from Planetary Nebulae. The Interstellar Medium has been found to be more inhomogeneous in the past epochs of the Galaxy than in the present time.