A number of new features appear in elastic collisions between atoms when one of the colliding systems is a positive ion. An account is given fist of some of these features, due to the long-range polarization force, which arise whether or not the ion and atom are of the same species. When they are of the same species symmetrical charge transfer may occur without change of internal energy and this leads to a number of further interference effects, the nature of which are described in general semi-classical theoretical terms. An account is then given of experiments which have confirmed the nature and existence of these effects.