  Recent studies of spectral lines of the CNO elements in the Sun and unevolved
disk and halo stars show that most published abundance data may be affected by
systematic errors due to inadequate 1D modelling of stellar atmospheres. This
raises doubts about previously derived trends of CNO abundances as a function
of [Fe/H]. In this review we concentrate on abundance ratios derived from weak
spectral features with similar dependences on temperature and pressure, which
ratios are then relatively insensitive to 3D model atmosphere effects and to
effective temperature and surface gravity. The recent controversy about the
trend of [O/Fe] vs. [Fe/H] is discussed, new results for C/O ratios as
determined from forbidden lines and high excitation neutral lines are
presented, and N/C ratios derived from NH and CH lines are commented on.
