  Planetary nebulae (PN) are an excellent laboratory to investigate the
nucleosynthesis and chemical evolution of intermediate mass stars. In these
objects accurate abundances can be obtained for several chemical elements that
are manufactured or contaminated by the PN progenitor stars, such as He, N, C,
and also elements that were originally produced by more massive stars of
previous generations, namely O, Ne, Ar, and S. Some of these elements are
difficult to study in stars, so that PN can be used in order to complement
results obtained from stellar data. In the past few years, we have obtained a
large sample of PN with accurately derived abundances, including objects of
different populations, namely the solar neighbourhood, the galactic disk and
anticentre, the galactic bulge and the Magellanic Clouds. In this work, we
present the results of our recent analysis of the chemical abundances of He, O,
N, S, Ar and Ne in galactic and Magellanic Cloud PN. Average abundances and
abundance distributions of all elements are determined, as well as
distance-independent correlations. These correlations are particularly
important, as they can be directly compared with the predictions of recent
theoretical evolutionary models for intermediate mass stars.
