Coupling matter (atoms, molecules, or solid-state materials) to the quantized electromagnetic 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). Unlike the traditional coherent control strategies, polariton chemistry does not rely on fragile electronic coherence and is robust to decoherence. Compared to the classical laser-matter interactions which operate with a large number of photons, cavity QED enables the hybrid system to initiate chemical reactions even without photons initially present in the cavity. Thus, polariton chemistry 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. Simulating the time-dependent polariton quantum dynamics of the hybrid matter-field systems is often a necessary and essential task, as these polariton photochemical reactions often involve a complex dynamical interplay among the electronic, nuclear, and photonic degrees of freedom (DOFs). However, accurately simulating the polaritonic quantum dynamics remains a challenging task and is beyond the paradigm of traditional photochemistry, which does not include quantized photons, and quantum optics which does not have a well-defined theory to include the influence of nuclear vibrations. Over the past years, enormous progress has been made to address this interdisciplinary challenge. We have witnessed how electronic structure theory (Sec. 3), non-adiabatic quantum dynamics (Sec. 4), and statistical mechanics (rate constant theory, in particular, Sec. 5) have actively participated in this exciting field in the past few years.