We describe here recent work on the electronic properties, magnetoexcitons and valley polarised electron gas in 2D crystals. Among 2D crystals, monolayer MoS2 has attracted significant attention as a direct-gap 2D semiconductor analogue of graphene. The crystal structure of monolayer MoS2 breaks inversion symmetry and results in K valley selection rules allowing to address individual valleys optically. Additionally, the band nesting near Q points is responsible for enhancing the optical response of MoS2. We show that at low energies the electronic structure of MoS2 is well approximated by the massive Dirac Fermion model. We focus on the effect of magnetic field on optical properties of MoS2. We discuss the Landau level structure of massive Dirac fermions in the two non-equivalent valleys and resulting valley Zeeman splitting. The effects of electron-electron interaction on the valley Zeeman splitting and on the magneto-exciton spectrum are described. We show the changes in the absorption spectrum as the self-energy, electron-hole exchange and correlation effects are included. Finally, we describe the valley-polarised electron gas in WS2 and its optical signature in finite magnetic fields.