The emergence of layered 2D semiconductors and especially the transition metal dichalcogenides (TMDCs) has provided a whole new perspective to the field of confined electron systems in semiconductors. This class of materials is comprised of weakly-interacting buckled atomic layers with sizable spin-orbit coupling. In recent years some unique observations and theoretical predictions uncovered the rich physics possessed by these materials. A review of these interesting findings will be provided in this talk including the coupling of spin to valley and the relation of inversion symmetry with the electronic structure. Transport and device applications will be discussed from the perspective of the sources of charge carriers in pristine crystals and their association with defects and traps. Finally, a report on recent trends in mediation of such defects and the challenge exquisite doping of TMDCs will be discussed.