The advent of efficient light-confining platforms marked the burgeoning of the field of Cavity Quantum Electrodynamics (Cavity QED), which brought along the realization of exotic phenomena that emerges under extreme light-matter interaction regimes.On the other hand, in chemistry, the interaction between light and matter is at the core of invaluable spectroscopic techniques and a number of photo-induced chemical processes. From a theoretical standpoint, light-matter interaction in those scenarios can be treated perturbatively, such that molecules and light preserve their individual identity. This picture breaks down when we consider one photonic mode interacting with molecules such that the energy of this interaction surpasses their respective linewidths. This so-called strong coupling (SC) regime has been accomplished in microsetups which can sustain confinement comparable to the wavelength of light that promotes molecular excitations. Under the SC regime, the excitations sustained by the device are no longer of purely material character but rather exhibit a hybrid molecular and photonic nature, and are usually known as polaritons. The recent observation of the slowing down of chemical reactions in the SC regime has incentivized communities of different backgrounds to converge in order to understand the range of possibilities the regime has to offer on the control of molecular processes. In this dissertation, a combination of quantum optics and chemical rate theory is used to understand the emergent dynamics of molecules embedded in an strongly confined electromagnetic environment. More specifically, an analysis of the tunability of chemical reactivity of realistic polariton setups in the dark (i.e. in the absence of any driving source) is presented. Later on, it is shown that Singlet Fission and Reverse-Intersystem Crossing, two photophysical processes of technological relevance, represent an ideal testing ground to explore the rich dynamics afforded under the polariton regime. In the latter phenomena, special emphasis is cast upon the fleeting nature of the electro- magnetic environment, as well as on its implications on the degree of control on molecular processes.