  We discuss the evolution of oxygen, carbon and nitrogen in galaxies of
different morphological type by adopting detailed chemical evolution models
with different star formation histories (continuous star formation or
starbursts). We start by computing chemical evolution models for the Milky Way
with different stellar nucleosynthesis prescriptions. Then, a comparison
between model results and ``key'' observational constraints allows us to choose
the best set of stellar yields. Once the best set of yields is identified for
the Milky Way, we apply the same nucleosynthesis prescriptions to other spirals
(in particular M101) and dwarf irregular galaxies. We compare our model
predictions with the [C,N,O/Fe] vs. [Fe/H], log(C/O) vs. 12+ log(O/H), log(N/O)
vs. 12+ log(O/H) and [C/O] vs. [Fe/H] relations observed in the solar vicinity,
along the disk and in other galaxies. By taking into account the results
obtained for all the studied galaxies (Milky Way, M101, dwarf galaxies and
DLAs) our main conclusions are: a) once the available observational data are
properly interpreted, there is no compelling evidence for requiring the
production of primary N in massive stars (M>10Msun); b) both C and N we see
today in the ISM come mainly from low- and intermediate-mass stars. In
particular, there is no need to invoke strong stellar winds in massive stars in
order to explain the evolution of C/O ratio in the solar neighborhood, as often
claimed in the literature (abridged).
