Microcavities are semiconductor, metal, or dielectric structures providing optical confinement in one, two or three dimensions. At the end of the 20th century, microcavities have attracted attention due to the discovery of a strong exciton-light coupling regime allowing for the formation of superposition light-matter quasiparticles: exciton-polaritons. In the following century several remarkable effects have been discovered in microcavities, including the Bose-Einstein condensation of exciton-polaritons, polariton lasing, superfluidity, optical spin Hall and spin Meissner effects, amongst other discoveries. Currently, polariton devices exploiting the bosonic stimulation effects at room temperature are being developed by laboratories across the world. This book addresses the physics of microcavities: from classical to quantum optics, from a Boltzmann gas to a superfluid. It provides the theoretical background needed for understanding the complex phenomena in coupled light-matter systems, and it presents a broad overview of experimental progress in the physics of microcavities.