In the current era, leading investigators have paid attention to certain kinds of materials. Of these, one of the potential materials “Transition Metal Chalcogenides” (TMDs) has been recognized to possess a two-dimensional configuration beyond the most efficient graphene. The initial theoretical and experimental innovations of the TMDs have predicted that they can be considered one of the key materials for future potential applications. Yet, very limited books are available on recently invented TMD materials, therefore, herein an effort is made to present basic concepts of the TMDs in an easy scientific manner for the common reader. This chapter is mainly devoted to a historical overview and fundamentals of the TMDs including, two-dimensional materials and their classifications, chalcogenide materials with their basic properties such as semiconducting lone-pairs, alternation of the valence electron pairs, donor–acceptor bonds, hybridization of bonds, multiple cantered bonds, and also define how TMDs are distinct from the usual chalcogenide materials by providing the description of the materials chemistry of the transition metals, theory of the valence bonds, ligand-field approach, crystal-field approach, and band structure.