In this review, we concentrate on an aspect of the physics of ion tails stimulated by observations of Comet Kohoutek (1973f), namely, the nature of moving structures in the tail. These motions could be bulk motions themselves or waves moving at the information speed of the medium; the resolution of this question from simple photographic data alone may not be possible.

We do not discuss the principal problem of ion tails, namely, the physical mechanism responsible for the creation of CO+ ions. Indeed, results from studies of gas phase chemistry presented at this Colloquium indicate that the solution may be quite complex. Also, we do not discuss either the details of the solar-wind comet interaction or dust tails.