The chemical and physical properties of transition-metal dioxides (TMDOs), transition-metal dichalcogenides (TMDCs) and specifically group IVB–VIA monolayers depend on the number of d electrons or the d-electron countD-electron count and thus on the group number in the periodic table, which controls possible chemical coordination in 2D space. The 2D compounds of Ti, Zr, and Hf and the group–VIA elements O, S, Se, and Te have found increasing interest. Group–IVB dichalcogenides have only sparsely filled d bands, and the Fermi levelFermi level usually is situated between the filled σ band of the chalcogen and the empty d bands (d0 compounds). The most stable monolayer structures consist of a hexagonal sublayer of metal atoms sandwiched between two chalcogen sublayers arranged as X–M–X with covalent-ionic M–X bonds. The two most important stable polymorphs, the trigonal prismatic structure (1T) and the octahedral structure (1H), realize such a three-sublayer arrangement. Owing to the different electronegativitiesElectronegativities, charge transfer occurs from the metal to the chalcogen atoms with greater electronegativity, causing the bonds to be partially ionic and in few cases strongly ionic. The thickness of the three sublayers that form the monolayer is typically 0.6–0.7 nm. Most TMDC monolayers are semimetals or semiconductors with modest band gaps, except for TiSe2 and tellurides. With decreasing electronegativity, when going down the corresponding column in the periodic table, the band gap decreases, and the tellurides are semimetalsSemimetals because the chalcogen-derived valence band overlaps with the transition metal d-orbital-derived conduction band. Potential applications are based on the modest or large band gaps and large electron mobilities of TMDCs. Potential applications include catalysis, nanoelectronics, optoelectronicsOptoelectronics, energy conversion, and energy storage devices. Most of these compounds are nonmetallic and nonmagnetic. The Janus structures of group-IVB TMDCs, such as ZrSSe, ZrSeTe, ZrSTe, HfSSe, HfSeTe, and HfSTe, are of increasing interest due to the loss of inversion symmetry. In non-centrosymmetric structures with two different chalcogen sublayers piezoelectricityPiezoelectricity emerges along with robust electron mobility.