Publisher Summary This chapter provides an overview of the dithiadiazolylium and dithiadiazolyl chemistry. It describes the major achievements and progress made in this field. Dithiadiazolyls belong to a class of heterocyclic sulfur-nitrogen free radicals. The X-ray analysis of many S3N2+. salts show oligomeric structures in which two S3N2+. units are bonded together through the overlap of singly occupied molecular orbitals (SOMOs) at sulfur. The dimer is linked by weaker secondary interactions to the anion and also to other S3N2+. dimer pairs. It is noted that much of the early work on dithiadiazolyls and their cationic dithiadiazolylium involved their synthesis, the recent high-yield routes to these materials from simple precursors has facilitated more extensive studies of their physical properties, particularly in the development and design of organic metals. The physical properties of these materials are particularly dependent on the solid-state packing, and consequently a minor modification of the molecular architecture can have major effects on the electronic response. In specific, the chapter provides a systematic background for the reactions, properties, and potential applications of these materials.