A survey is given of experimental and theoretical studies related to the fundamental properties of single crystal vanadium oxides. Emphasis is put on the relationship which exists between V2O5 and the lower oxides, mainly V4O9 and V6O13. Special attention is paid to the role played by the vanadyl—oxygen vacancy, both in the transition mechanism of V2O5 into its lower oxides and in the surface properties of these materials. The role of the lower oxides in catalytic oxidation reactions is commented upon.