Resumen
We present a large and homogeneous sample of optical and UV observations of PNe and derive their abundances and the implications for the Intermediate Mass Stars (IMS) and Galactic chemical evolution. We observe the ``hot bottom burning'' (HBB) reaction, oxygen depletion (ON cycle) and/or production (likewise carbon) during the third dredge-up. All these effects are more efficient at lower metallicity. A strong warning is that oxygen can not be used to derive the initial composition of the progenitor star, but we have to use other elements like sulfur or argon.
