The problem of electrons in 2D is one of the most important topics in contemporary condensed matter physics. Coulomb interactions between charge carriers in 2D are dramatically enhanced with the much-reduced dielectric screening compared to their bulk counterpart. Recent advances in the development of atomically thin layers of materials have opened up new opportunities for the study of many-body effects in 2D. In the last talk, we will discuss the observations of strong excitonic effects in graphene and in a valley Hall semiconductor through optical spectroscopy. We will demonstrate the control of Coulomb interactions in such atomic membranes by tuning their dielectric screening through an electrostatic gate. As a result of such strong interactions, optically excited electrons are rapidly thermalized, which can be directly monitored in the femtosecond time-scale. The role of electron-phonon coupling and the importance of polaronic effect in graphene, a nonpolar crystal, will also be discussed.