Since its introduction into organic chemistry more than fifty years ago, RuO₄ has been used mainly as a strong oxidizing agent. Owing to its high reactivity, catalytic reactions employing RuO₄ were often considered to be sluggish or unselective. However, within the past ten years, several groups have reported the development of new and selective oxidative transformations possible only under RuO₄ catalysis. The present article summarizes the state of research in this field and tries to give a systematic overview of the reactivity and the reaction mode of RuO₄. In the final section, relative reactivities between olefins or olefins and functional groups are discussed. The information provided in the present paper might serve as a good starting point for chemists interested in applying RuO₄-catalyzed reactions in organic synthesis or methodology.