The element selenium has provided a fertile field of study, stemming from its many roles and applications in biology, materials chemistry, and synthesis.¹  The organic chemistry of selenium is quite distinct from that of sulfur and tellurium, and often leads to unexpected and valuable types of reactivity,²  of which redox reactions account for much of its behaviour. The range of oxidation states of selenium, from −2 to +6, and their facile interchange has enabled a variety of useful oxidation and reduction processes. The deployment of selenium and its compounds in a catalytic fashion mitigates some of the concerns related to its potential toxicity, lowers the environmental impact of such processes, and decreases the cost of using these materials, particularly for large-scale applications.^3,4  The biological and materials aspects of the element, including their related redox processes, are covered in other chapters of this book, while the present chapter describes catalytic transformations that are of particular interest in green organic synthesis.