The chapter presents the progress made in recent years (mostly after 1987) in the chemistry of the selenium and tellurium compounds in oxidation states from 3 to 6 but, to make the presentation more comprehensible, it also contains selected references to earlier achievements. It is divided into seven sections, comprising particular classes of organoselenium and organotellurium derivatives which are discussed in the order of increasing oxidation states: selenonium and telluronium salts, selenoxides, telluroxides, selenimines and tellurimines, selenones, tellurones and selenoximines. All the sections are divided into subsections devoted to the synthesis and structure, reactivity and transformations and synthetic applications of the discussed classes of compounds. Particular emphasis is laid on the synthesis and properties of the chiral, non‐racemic forms of each class of compounds.