Phytohormones are very much involved in directing the plant growth, in a coordinated fashion in association with metabolism that provides energy and the building blocks to develop the form that we recognize as plant. Out of the recognized hormones, attention has largely been focused on Auxins, Gibberellins, Cytokinins, Abscisic acid, Ethylene and more recently to Brassinosteroids. However, in this book we are providing the information about a natural chemical, Salicylic Acid, that could be raised to the status of the above phytohormones because it has significant impact on various aspects of the plant life.Salicylic acid (SA) was first discovered as a major component in the extracts from Salix (willow) whose bark from ancient time, was used as an anti-inflammatory drug. This acid (SA) is a phenol, ubiquitous in plants generating a significant impact on plant growth and development, photosynthesis, transpiration, ion uptake and transport and also induces specific changes in leaf anatomy and chloroplast structure. SA is recognized as an endogenous signal, mediating in plant defence, against pathogens. This book includes contributions made by various experts, spread over the world. Since the work on Salicylic Acid is currently rather scattered it was thought prudent to draw this all together in one volume. This will naturally facilitate a ready reference of the material to students and researchers, alike.