Long considered as structural curiosities, the potential of hypervalent halogen reagents in organic synthesis was first recognized in oxidation reactions, where they could replace toxic metal-based oxidants. In the last three decades, the potential of hypervalent halogen reagents for more complex functional group transfer reactions has been fully realized and has contributed to making them an essential tool in modern organic synthesis. This Science of Synthesis volume serves as an essential reference for newcomers in the field and aims to help orientate them in this rapidly growing area of modern synthetic chemistry. Rather than being comprehensive, the coverage presents selected important conceptual advances and practical procedures both for the synthesis of hypervalent halogens (e.g., λ³-bromanes and chloranes, water-soluble hypervalent iodine reagents, benziodazoles, benziodoxoles, and benziodoxolones) and their use in synthetic transformations (e.g., aromatic halogenation, C-N bond formation, asymmetric synthesis, photochemistry, electrochemistry, and catalysis)