The group of XB transition-metal dioxide (TMDO)Transition-Metal Dioxide (TMDO) and XB transition-metal dichalcogenide (TMDC)Transition-Metal Dichalcogenide (TMDC) monolayers with MX2 stoichiometry consist of a central layer of transition-metal atoms, M = Ni, Pd, and Pt sandwiched between two layers of chalcogenide atoms, X = O, S, Se, and Te. Among the dynamically, thermally, and mechanically stable structures, the octahedral T phase usually forms the most stable structure. In addition to the less important metastable H phase, a pentagonal orthorhombic phase with lower symmetry and strongly anisotropic properties has attracted increasing amounts of attention from researchers. Nickel, an inexpensive group-XB transition metal with a similar d-electron configuration as the noble metals Pd and Pt, is of interest as an alternative material. Nickel oxide and Ni dichalcogenides are relevant as electrode materials for electrochemical energy storage. Noble-type TMDCs with almost full occupation of d orbitals exhibit attractive properties, such as largely tunable electronic structures. Therefore, these materials may be useful for electronics, optoelectronicsOptoelectronics, catalysis, and sensors. Pentagonal PdSe2 and hexagonal PdTe2 monolayers and their few-layer sheets are receiving increasing attention. Owing to the low symmetry of pentagonal PdSe2, new properties are possible, and for hexagonal PdTe2 the combination of superconductivity with topical nontrivial states has attracted the interest of researchers. Several TMDCs and their Janus compounds are promising thermoelectric materials that can serve as alternatives to conventional devices, which are based on the Seebeck effect, using a temperature gradient to obtain clean energy.