Tush, my good lord, this superficial tale Is but a preface of her worthy praise; King Henry the Sixth: Part I, Act V, Sc. 5 This volume is the direct result of a NATO. Advanced Study Institute (ASI) of the same title, held at Bishop's University, Lennoxville, Quebec, Canada in August 1991. All the major presentations had been commissioned, so to speak, during the organisational phase. This was done with the view of not only having a structured ASI, whIch is expected to be a high-level tutorial activity, but also the ensuing volume. As will be indicated in the general introduction, there have been a couple of meetings and publications dealing with rhythms in fishes in the past twenty years. However, as in other disciplines, there has been substantial progress in this field also. Further, I wished to cover almost all aspects and come out with a volme which will be as complete as possible. Of course, this failed to materialise. In spite of starting the arrangements two years before the event, several lecturers were not able to attend due to a number of reasons. Some could be replaced while others could not Taking into account their individual specialties, I asked about twenty active workers in the field to provide provoking overviews, not simply reviews of their own work. Also, this being a NATO.