  This paper summarizes a project designed to study abundances in a sample of
planetary nebulae representing a broad range in progenitor mass and
metallicity. We collect abundances of C, N, and O determined for the entire
sample and compare them with theoretical predictions of planetary nebula
abundances from a grid of intermediate-mass star models. We find very good
agreement between observations and theory, lending strong support to our
current understanding of nucleosynthesis in stars with progenitor masses below
8 solar masses. This agreement between observation and theory also supports the
validity of published stellar yields of C and N in the study of the abundance
evolution of these two elements.
