Jasmonic acid (JA) is an essential phytohormone for plant development and stress response. It not only acts as the regulatory molecule in root elongation, germination, pollen production, fruit ripening and plant senescence, but also acts as the signaling compound activating plant defenses against pathogens, herbivory, wounding and abiotic stress. This book discusses the biosynthesis, functions and the role jasmonic acid has in plant development.