  In this review we present a brief discussion on the observational evidence in
favor of the presence of temperature variations, and conclude that many
planetary nebulae show spatial temperature variations that are larger than
those predicted by 1D static chemically homogeneous photoionization models. To
determine accurate chemical abundances it is necessary to know the cause of
these temperature variations and several possibilities are discussed. The
importance of this problem is paramount to test the models of stellar evolution
of low and intermediate mass stars and of the chemical evolution of galaxies.
We conclude that the proper abundances for chemically homogeneous PNe are those
derived from recombination lines, while for the two-abundance nebular model the
proper heavy element abundances relative to hydrogen are those derived from
visual and UV collisionally excited lines adopting the t2 values derived from
Te([OIII]) and Te(Balmer)
