This text discusses inhibitors and phytohormones and the practical applications for achieving a more sustainable agriculture. The work focuses on the functions of various groups of active molecules, their direct effect upon plant growth, but also implications for their impact upon the surrounding environment are explored. The main idea of the book evolved for the need to determine a balance among natural growth inhibitors and phytohormones. This approach was pursued through a better understanding of their biochemical pathways, their effects on plants physiological functions, and their influence upon stress factors on plant ontogenesis. Therefore, this effort proposes a more logistic approach to the study of plant physiology, in which the plant-soil interactions are discussed, with a profound description of different allelochemicals and their effects on plants growth.