Coupling molecules to the quantized radiation field inside an optical cavity creates a set of new photon-matter hybrid states, so-called polariton states. These polariton states have delocalized excitations among molecules and the cavity mode, which have been shown to facilitate new chemical reactivities. Theoretical investigations play a crucial role in understanding new principles in this emerging field and have suggested interesting reaction mechanisms enabled by cavity Quantum Electrodynamics (QED). Polariton chemistry thus provides a new strategy to control chemical reactivity in a general way by tuning the fundamental properties of photons and provides a new paradigm for enabling chemical transformations that can profoundly impact catalysis, energy production, and the field of chemistry at large.