This set of lecture notes provides a detailed and up-to-date description of a field undergoing explosive growth, that of confined photon systems in the shape of microcavities or photonic crystals. Bringing together world leaders in the field, it provides all the basic tools needed to master a subject which will have both major impact in fundamental studies and widescale applications. Confined photon systems enable the study of low-dimensional photonic systems, modified light-matter interaction, e.g. between excitons and photons in all-solid-state semiconductor microcavities, and of many phenomena of quantum optics, including single photon generation, squeezed light, quantum state entanglement, non-local quantum measurements, and, potentially, quantum computation. They are also on the verge of yielding new, high performance optical devices for large-scale industries such as telecommunications and display technology.