Information available on the coordination of boron in alkali borate glasses is reviewed briefly together with recent evidence from studies of dilute solutions of transition metal ions in silicate and borate glasses. The variations of oxidation-reduction equilibria with alkali content suggest that “complexes” must be recognised in the glasses. The nature of these complexes is discussed, and the change from one type of complex to another is considered for Co II, Ni II, Fe III and Cr VI ions. It is then emphasized that the formation of tetrahedral boron complexes, tetrahedral aluminium complexes and transition metal ion complexes is essentially a problem in chemical thermodynamics. Finally the change in properties such as thermal expansion and UV cut-off at about 15 mol% Na 2O is related to a possible change in the sodium ion coordination.