Crystals consisting of atoms of an inert gas (or of gas molecules) and containing imbedded foreign atoms, radicals, or ions are investigated and their parameters are discussed. A review is given of studies of the diffusion of the trapped particles in a given matrix and of the spectroscopic investigations of these particles. An analysis is given of the properties of gas crystals containing imbedded particles at the crystal lattice sites and having long lifetimes at low temperatures.