A "biological clock" has now been inferred in so many and such diverse organisms and tissues that even a summary of the more interesting and important observations would be a tedious and encyclopaedic compilaΒ tion, whose bibliography would assume a daunting size. It would also be obsolescent on the day of publication. The new titles appearing in the monthly lists are scattered through many journals, but a new journal devoted exclusively to rhythm research published its first issue in May, 1970-the Journal of Interdisciplinary Cycle Research-and another, Chronobiology, appears in 1973. In this volume several authors have been asked to review separate aspects within their own fields of study, in the hope that thereby the reader might gain an idea of the many directions of active progress and be better placed to interrelate them than would be possible after a more exhaustive study of a limited part of the field. The outcome is a series of essays in which each contributor has exercised his individuality in ideas, style and presentation, and, at some points, in vocabulary, although the glossary includes a number of terms which have been fairly generally used.