This review summarizes recent theoretical advances in polariton chemistry, and it is organized as follows. Section 2 discusses the fundamental theoretical framework behind light-matter interactions. Starting from the most rigorous Hamiltonian, it discusses how and when to perform various approximations to reduce the computational complexity while keeping the relevant physics. Section 3 discusses how to apply the fundamental framework of the previous section to realistic systems with ab-initio electronic structure methods. This section reviews different methods of marrying electronic structure methods to these hybrid light-matter systems to model complicated polariton systems. Section 4 applies the methods of Sections 2 and 3 to photochemistry, showing how simple chemical reactions such as photoisomerization or charge transfer reactions can be altered by strongly coupling electronic transitions to a cavity. Section 5, similarly, summarizes recent progress in understanding vibrational strong coupling (VSC), where the nuclear vibrational states are strongly coupled to the cavity, leading to changes of the ground state chemical kinetics. This section further shows how the fundamentals of statistical mechanics like rate constant theory can be used to understand these reactions. Section 6 goes on to present recent theoretical explanations of experiments in the collective coupling regime, a regime that is largely mysterious since direct modeling of experimentally relevant numbers of molecules is typically impossible, and simple models like the TC model break down for experimentally realizable coupling strengths. This section also discusses various recent theoretical hypotheses to explain the experimentally observed suppression or enhancement of the reaction rate constant under the collective vibrational strong coupling regime.