This volume is one of those published from the proceedings of the invited lectures to the First International Congress of Comparative Physiology and Biochemistry I organized at LI~e (Belgium) in August 1984 under the auspices of the Section of Comparative of the International Union of Biological Sciences. In a Physiology and Biochemistry general foreword to these different volumes, it seems to me appropriate to consider briefly what may be the comparative approach. Living organisms, beyond the diversity of their morphological forms, have evolved a widespread range of basic solutions to cope with the different problems, both organisma! and environmental, with which they are faced. Soon after the turn of the century, some biologists realized that these solutions can be best comprehended In the framework of a comparative approach integrating results of phYSiological and biochemical studies done at the organismic, cellular and molecular levels. The development of this approach amongst both physiologists and biochemists remained, however, extremely slow until recently.