Evidence for non-homogeneity in the metal abundances of giant stars in various globular clusters is reviewed. The nonhomogeneity is of two kinds: (1) differences in the value of from star to star in the same cluster, for particular values of A, and (2) non-constancy of the cluster mean value of as a function of A. Observational evidence for the existence of variations of both kinds is cited, and discussed for four “nucleosynthetic” groups of elements: viz., s-process, Fe-peak, α-process, and CNO. The rival hypotheses of core-envelope mixing and primordial abundance variations are examined in the light of the observations.