In its simplest manifestation, the Casimir effect is a quantum force of attraction between two parallel uncharged conducting plates. More generally, it refers to the interaction — which may be either attractive or repulsive — between material bodies due to quantum fluctuations in whatever fields are relevant. It is a local version of the van der Waals force between molecules. Its sweep ranges from perhaps its being the origin of the cosmological constant to its being responsible for the confinement of quarks.This monograph develops the theory of such forces, based primarily on physically transparent Green's function techniques, and makes applications from quarks to the cosmos, as well as observable consequences in condensed matter systems. It is aimed at graduate students and researchers in theoretical physics, quantum field theory, and applied mathematics.