  Atomically thin group-VIB transition metal dichalcogenides (TMDs) have
recently emerged as a new class of two-dimensional (2D) semiconductors with
extraordinary properties including the direct band gap in the visible frequency
range, the pronounced spin-orbit coupling, the ultra-strong Coulomb
interaction, and the rich physics associated with the valley degree of freedom.
These 2D TMDs exhibit great potentials for device applications and have
attracted vast interest for the exploration of new physics. 2D TMDs have
complex electronic structures which underlie their physical properties. Here we
review the bulk electronic structures in these new 2D materials as well as the
theoretical models developed at different levels, along which we sort out the
understandings on the origins of a variety of properties observed or predicted.
