Open channels exist between the rows or planes of atoms in a crystal. In experiments with carefully defined beams of energetic ions it is possible to inject particles into these channels. There they travel with relative ease, being reflected back and forth between the channel walls by a correlated series of atomic collisions. The main types of experiment on this phenomenon are summarized and the classical theory is developed. Classical treatment is not adequate for light particles, such as electrons or positrons because their de Broglie wavelength is greater and a wave mechanical version is outlined in a form which is also suitable for describing protons. This confirms that for heavy particles classical theory can explain any experiments performed so far, but it is just possible that the interpretation of more precise proton experiments may require wave mechanics.