Book excerpt: This book provides a pedagogical introduction to the emerging field of Polariton Chemistry, where optical cavities are utilized to control the physicochemical properties and dynamics of molecular systems. Given the early stages of this interdisciplinary research area, it is important to provide a common language and starting point for interested researchers across Chemistry, Physics, and Engineering This edited compendium fills a void given that there is currently no analogue in the current literature. Topics covered include Single-Molecule Strong Light-Matter Coupling; Collective Strong Light-Matter Coupling; and Ultrastrong Light-Matter Coupling