Single‐ and few‐layer transition metal dichalcogenides have recently emerged as a new family of layered crystals with great interest , not only from the fundamental point of view, but also because of their potential application in ultrathin devices. Here the electronic properties of semiconducting MX2, where M =Mo or W and X = S or Se, are reviewed. Based on of density functional theory calculations, which include the effect of spin‐orbit interaction, the band structure of single‐layer, bilayer and bulk compounds is discussed. The band structure of these compounds is highly sensitive to elastic deformations, and it is reviewed how strain engineering can be used to manipulate and tune the electronic and optical properties of those materials. Further, the effect of disorder and imperfections in the lattice structure and their effect on the optical and transport properties of MX2 is discussed. The superconducting transition in these compounds, which has been observed experimentally, is analyzed, as well as the different mechanisms proposed so far to explain the pairing. Finally, a discussion on the excitonic effects which are present in these systems is included. The recent isolation of atomically thin films of semiconducting transition metal dichalcogenides MX2(M = Mo, W and X = S, Se) has opened new opportunities for the application of two‐dimensional crystals in digital electronics, spintronics and optoelectronics devices. This family of semiconductors presents a number of remarkable features such as spin‐valley coupling, superconductivity, excitonic effects etc., which dramatically depend on their thickness (number of layers) and which are briefly reviewed in this article.