These lectures focus on the properties of elementary optical excitations in two-dimensional degenerate electron or hole systems. Transition metal dichalcogenide monolayers and their van der Waals heterostructures constitute an ideal platform to explore this physics. After a brief review of the material system, we will develop a formalism for analyzing exciton-electron as well as polariton-electron interactions. We will then describe how external electric fields could be used to achieve tunable quantum confinement of excitons. When an electron-doped van der Waals heterostructure is embedded inside a microcavity, the interactions between exciton-polaritons could be enhanced, rendering this system of potential interest for exploring strongly correlated photonic systems.