This book grew out of a passion for rhythmic phenomena in biological systems, which has impelled my work for 25 years. I had the chance of being introduced to that field when I joined the group of Ilya Prigogine at the University of Brussels, for what I thought would be a one-year experience at the end of my studies in chemistry. I was rapidly hooked, however, and still have the pleasure of pursuing my research there. I would like to seize this opportunity to express my gratitude to Ilya Prigogine, who kindly agreed to write a preface (reproduced here as a foreword) for the French edition of this book; I have been marked by the enthusiasm that drives his incessant questioning of the world. The atmosphere in Brussels has always been ideal for developing my work; in addition to Ilya Prigogine and Paul Glansdorff, I am indebted to Gregoire Nicolis and Rene Lefever for making this friendly intellectual climate so fruitful.

This book owes much to the graduate students and co-workers with whom I had the pleasure of collaborating over the years: Jean-Louis Martiel, Olivier Decroly, Federico Moran, Yue-Xian Li, Genevieve Dupont, and Jose Halloy. They made decisive contributions to the various topics considered in this book, by actually carrying out much of the research described. Special thanks are due to Thomas Erneux, with whom I am always happy to pursue new ventures.