Plant ecophysiology is concerned fundamentally with the physiology of plants as modified by fluctuating external influences. According to a definition agreed by the Societe d'Ecophysiologie, ecophysiology "involves both the descriptive study of the responses of organisms to ambient conditions and the causal analysis of the corresponding ecologically dependent physiological mechanisms, at every level of or­ ganization. The ecophysiological approach must take into account polymorphism" (that is, structural and functional diversity) "in in­ dividual responses, which are largely responsible for the adaptive capacity of any given population. In this respect, ecophysiological study yields information which is fundamental for an understanding of the mechanisms underlying adaptive strategies" (Vannier 1994). of the author is to convey the conceptual framework upon The aim which this discipline is based, to offer insights into the basic mechanisms and interactions within the system "plant and environ­ ment", and to present examples of current problems. Although physi­ ological ecology is currently an exciting and expanding branch of science it is by no means a recent field of research. I have tried to por­ tray its rich historical background in the choice of illustrations and tabular material; the results presented reflect the broadness of vision, the struggles and successes of the pioneering experimental eco­ physiologists, as well as the most recent advances in knowledge.