The study of water stress is one of the most interesting subjects in. the investigation of water relations in plants. From the theoretical point of view it is concerned with investigating the mechanisms of the distribution and movement of water in the plant organism and the way in which physiolo­ gical processes are influenced by water deficiency. From the practical point of view, water deficiency is a major factor limiting plant production. It has been progressively shown that water deficiency is not by far· only a factor in plant life in dry climates, that obvious wilting is not the first warning sign of water deficiency and that moderate water stress, caused by temporary negative water balance during the day, affects physiological ac­ tivity and decreases prodnction in the ecological conditions of the temperate zone. In addition, even general water deficiency is not today confined to arid or semi-arid zones and to the absolutely dry season of the year. The tremend­ ous consumption of water in our civilization has become today, even in the temperate zone, an important competitor with the plant cover. The study of water relations from the aspect of water stress is, therefore, important both theoretically and practically. I assume, therefore, that it was useful, important and interesting to meet in a symposium on water stress in plants and to discuss, as far as possible, in detail problems which are obviously among the main, whose solution would help plant physiology in increasing and improving plant production.