The monograph summarizes the current literature and the results of the authors' own studies on the influence of a number of external factors on the content of hormones in plants. The focus is on water scarcity in soil and air, salinity, temperature and mechanical damage. The processes due to which the concentration of hormones changes under the influence of external influences (their synthesis, transport, metabolic transformations) are considered. The importance of hormone concentration in the regulation of stomatal and hydraulic conductivities and their role in optimizing water balance, gas exchange and photosynthesis in changing environmental conditions were analyzed. The issues of hormonal regulation of the growth response to the influence of external factors are considered. There are presented data on the value and mechanism of hormonal regulation of cell extension ensuring growth maintenance under suboptimal conditions.