The importance of tides on bodies in the Solar System has been understood and quantified for many decades. The various contributions in this Volume reflect the maturity of this field of research. Advances in the appreciation of the role of tidal effects on planet/stellar evolution are also remarkable. As the extragalactic world is concerned, the situation is paradoxical. Whereas the effects of tidal forces are spectacular - they alter the morphology of the most massive galaxies and may lead to the total destruction of the dwarf satellite galaxies - , it is only in the seventies that tides were recognized as actors of galactic evolution. Observations of jet-like structures, antennas, bridges and plumes occurred well before they were interpreted as "tidal tails". Only the first numerical simulations of galaxy mergers convinced the community about the real nature of these stellar structures, whereas the straightforward consideration that galaxies are flaccid bodies might have lead to this conclusion much earlier. It is however true that the bulges generated by the Moon and the Sun on the Earth's oceans, which were interpreted as the result of tides soon after the laws of gravity were established, do not resemble the gigantic appendices that emanate from some galaxies although their origin is similar. What seems obvious now was not fifty years ago.
Having said that, it would be misleading to claim that tidal forces are the only actors of galactic morphological transformations. In fact, the fraction of mass involved in material that is tidally affected is relatively small. Other physical processes such as violent relaxation are more important in shaping galaxies. The nuclear starbursts often associated with galaxy mergers are not directly induced by tidal forces. Furthermore, not all the collisional debris found around mergers are, strictly speaking, of tidal origin. With these preliminary remarks, we wish to precise that this Review specifically focusses on tides in colliding galaxies and is not an overview of interacting galaxies and associated phenomena. For a more general insight on galaxy-galaxy collisions and mergers, the reader is referred to the somehow old but comprehensive reviews of Sanders & Mirabel (1996) and Schweizer (1998), dealing with observations, and Struck (1999), more focussed on simulations.
We will first present the historical context of the discovery of tails around galaxies, and detail how the role of tides became evident. The tremendous progress in the numerical modeling of tidal tails is detailed before a more theoretical and analytical approach of the formation of tidal tails is presented. In the following sections, we investigate the physical properties of tidal tails, emphasizing what deep multi-wavelength observations bring to their study. We then make close up on the tails, looking at their sub-structures: from young stars and star clusters to tidal dwarf galaxies. Finally, we examine what tidal features may tell us about galaxies: what they are made of, and how and when they were formed. We hope to convince the reader that tails are not only aesthetic add-ups in images of colliding galaxies but may be used to address fundamental questions of astrophysics.