Sactipeptides represent an emerging class of ribosomally synthesized and post-translationally modified peptides with diverse bioactivities. The common hallmark of sactipeptides is intramolecular thioether bond that crosslinks the sulfur atom of a cysteine and an α-carbon of an acceptor amino acid. This review summarizes recent achievements in many aspects of sactipeptides, including discovery, activity, biosynthesis, and mode of action, with a particular focus on the common enzymology of radical SAM chemistry leading to the unusual S-to-Cα thioether linkages.