  We previously determined abundances of He, C, N, O, and Ne for a sample of
planetary nebulae (PNe) representing a broad range in progenitor mass and
metallicity, and we now compare them with theoretical predictions of PNe
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 below 8Mo in birth mass. In
particular, C and N abundance patterns are consistent with the occurrence of
hot-bottom burning in stars above roughly 3.5Mo, a process that converts much
of 12C into 14N during the AGB phase. This agreement also supports the validity
of published stellar yields of C and N in the study of the abundance evolution
of these elements.
