Publisher Summary This chapter outlines the mechanistic understanding of organic oxidation chemistry that has been generated by using high-valent metal oxo compounds of technetium and rhenium. Oxidation of organic compounds and alkenes, in particular, is an enormous area, and transition-metal-based catalysts have played a pivotal role. The similarity of the chemistries of rhenium and technetium elements with those of Mo, W, Ru, and Os makes them attractive models for probing reaction mechanisms for compounds of metals in the middle of the transition series. The chapter describes the preparation, structure, and chemical behavior of methyl rhenium trioxide (MTO) and discusses the bishydroxylation of alkenes. Detailed mechanistic inquiries into the mechanisms of transformations has shed light on several long-standing problems in organic oxidation chemistry and has revealed the importance of important structural classes of metal oxo and peroxo complexes.