Selenium dioxide has developed considerable stature since it was first reviewed in this series. Search for examples of oxidation by selenium dioxide or selenious acid is difficult because the terms are seldom indexed. Frequently it is difficult to decide whether selenium dioxide or selenious acid was the oxidizing agent employed in a reaction because it is impossible to determine if water was present. The determination is complex because the free acid or anhydride may react reversibly or irreversibly with various solvents such as alcohols or acids. To give selenium-containing compounds of unknown structure which might be the real oxidizing gents. In this chapter, the practical aspects of oxidation with selenium dioxide are summarized. Mechanisms are treated in detail.