  We discuss the accuracy of the abundance determinations of H II regions in
our Galaxy and other galaxies. We focus on the main observational constraints
derived from abundance determinations that have implications for models of
galactic chemical evolution: a) the helium to hydrogen abundance ratio, He/H;
b) the oxygen to hydrogen abundance ratio, O/H; c) the carbon to oxygen
abundance ratio, C/O; d) the helium to oxygen and helium to heavy elements
abundance ratios, Delta Y/ Delta O and Delta Y/ Delta Z; and e) the primordial
helium abundance, Yp.
