This book represents the authors' lifetime dedication to the study of inhibitors and phytohormones as well as its practical applications for achieving a more sustainable agriculture. Their 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 from 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 holistic 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. A rigorous attention is also paid to discuss the role of microorganisms in ecosystems and their capability to synthesize physiologically active substances, which trigger also unique plant-microbial interactions. These synergies are leading scientists to the discovery of major breakthroughs in agriculture and pharmacology that are revolutionizing old epistemologies and thus, contributing to the emergence of a philosophy of interconnectedness for the whole biosphere.