This chapter covers the elemental monolayersElemental monolayers of Se, Te, and Po, which belong to the main group-VIA elements and evolve from the insulators O and S to the metal Po. The most stable bulk structures of the chalcogens selenium and tellurium consist of quasi 1D helical chainsHelical chains with only two chemical bonds per atom. Consistent with the increasing metallic character in this group, bulk polonium shows enhanced metallic behavior with a simple cubic structure. The inherent multivalencyMultivalency of these elements with variable oxidation statesOxidation states (+ 6 and – 2) extends the covalent bondingCovalent bonding types of Se and Te from low-dimensional chains to the formation of various metastable tetragonal, rhombic, and monoclinic 2D structures, usually derived by global structural searching combined with first-principles calculations. Note that tellureneTellurene is a 2D material first proposed by computational simulations. The most stable structure of poloniumenePoloniumene seems to be a trigonal three-layer assembly. In general, 2D bonding is weak in this group, and conflicting discussions concerning the existence and the thermodynamic, dynamic, and thermal stabilityThermal stability of some postulated phases are going on. Liquid-phase exfoliationLiquid-phase exfoliation and physical and chemical vapor deposition provide few-layer sheets of seleneneSelenene and tellureneTellurene with the trigonal bulk structure, whereasMolecular Beam Epitaxy (MBE) molecular beam epitaxyEpitaxy delivers monolayers of helical chain α tellurene and tetragonal β tellureneTellurene. The relevant features of multifunctional chalcogen monolayers and few-layer assemblies are tunable band gaps, high carrier mobilities, low thermal conductivity, strong angle-dependent response, and ambient stability. Potential applications of 2D chalcogens cover a wide range of possibilities, including batteries, photodetectorsPhotodetector, modulators, as well as photothermal and medical applications. Both seleneneSelenene and tellureneTellurene are suitable materials for application in field-effect transistors and in ultrascaled complimentary metal–oxide–semiconductor circuits. Furthermore, some monolayers are 2D topological insulators with gapped interior states and gapless metallic edge states, which are of interest in spintronicsSpintronics, optoelectronicsOptoelectronics, and thermoelectrics.