  Monolayers of transition metal dichalcogenides, namely, molybdenum and
tungsten disulfides and diselenides demonstrate unusual optical properties
related to the spin-valley locking effect. Particularly, excitation of
monolayers by circularly polarized light selectively creates electron-hole
pairs or excitons in non-equivalent valleys in momentum space, depending on the
light helicity. This allows studying the inter-valley dynamics of charge
carriers and Coulomb complexes by means of optical spectroscopy. Here we
present a concise review of the neutral exciton fine structure and its spin and
valley dynamics in monolayers of transition metal dichalcogenides. It is
demonstrated that the long-range exchange interaction between an electron and a
hole in the exciton is an efficient mechanism for rapid mixing between bright
excitons made of electron-hole pairs in different valleys. We discuss the
physical origin of the long-range exchange interaction and outline its
derivation in both the electrodynamical and $\mathbf k \cdot \mathbf p$
approaches. We further present a model of bright exciton spin dynamics driven
by an interplay between the long-range exchange interaction and scattering.
Finally, we discuss the application of the model to describe recent
experimental data obtained by time-resolved photoluminescence and Kerr rotation
techniques.
