The energy of an electromagnetic field can be stored with the help of confining devices, known as optical cavities, that localize it temporarily. If a dielectric material with a high dipole moment shares space with the confined field, the optical properties of the system correspond to neither of its components but rather to a light-matter hybrid, whose excitations are called polaritons. While experimental realizations of this phenomenon date back to decades ago, it was not until the last decade when advances in sample preparation technologies enabled the investigation of the consequences of polariton formation on the observables related to the material. In this regard, a particularly active area of interest is the study of chemical reactivity under strong-light matter coupling. A striking development in the area is the observation that reactions inside infrared cavities, which couple to bond vibrations, experience a change in their rate even without any energy source other than room temperature. This effect has been observed in various reactions, including organometallic and carbonyl substitutions, and even in enzymatic processes. Consistently, these experiments show that rate modification is subject to the same conditions as polariton formation: the requirement of resonance between cavity and vibration and the intensification of the effect with the concentration of the sample. However, cavity quantum electrodynamics (CQED), the same theory that has successfully explained and predicted the optical properties of polaritons for decades, at first glance suggests that a local process such as a chemical reaction should not be affected by an essentially delocalized phenomenon such as light-matter coupling.This dissertation presents how CQED combines with two approaches to chemical rate theory: adiabatic reactions described within transition state theory (TST), and non-adiabatic processes, described by Marcus' theory of charge transfer. In the first case, it is found that under typical experimental conditions, a description at the level of TST predicts that vibrational strong light-matter coupling should produce no effect on the chemical rate. In contrast, for non-adiabatic processes, it is possible to conceptualize rate alteration in terms of a modified distribution of activation energies that accounts for the presence of polariton modes. Additionally, this work presents a group-theoretical method to simplify the description of a collective of identical oscillators with an arbitrary structure to a cavity mode, which may have applications to understanding chemical processes and non-linear response phenomena.
