  Although globular clusters are generally chemically homogeneous, substantial
abundance variations are sometimes seen even among unevolved main sequence
stars, especially for the CNO group of elements. Multi-object
intermediate-dispersion spectroscopic systems are now being used to determine
the patterns of abundance variations for large samples of stars, while high
dispersion spectrographs on 8m-class telescopes are providing good spectra of
individual faint stars. In some circumstances the spectra of many similar stars
can be combined to yield extremely high S/N spectra. The sample of clusters
which has been studied remains small, but it seems that many of the more
metal-rich clusters must have experienced some sort of self-enrichment, either
in a proto-cluster stage, or through successive episodes of star formation or
some other processes early in their lives. The metal-poor clusters provide
equally clear evidence for internal mixing and dredge-up of processed material
within evolving red giant stars.
