Phytohormones are known to affect the growth and development of plant directly as well as indirectly. Salicylic acid (SA) is a phenolic phytohormone which induces systemic resistance in plants and also regulates defence responses. The derivatives of SA also play an important role in the regulation of various physiological and developmental processes in plants under normal and stressful environmental conditions. SA regulates seed germination, photosynthesis, ethylene biosynthesis, enzyme activities, nutrition, flowering, legume nodulation and overall growth and development of plant. Recently, advancement in elucidating the specific pathways of SA signal transduction has been noticed which helps in understanding the expression of specific genes associated with different developmental programs. The horizon of SA-mediated regulation of various physiological processes has also expanded, and various studies enumerating the efficacy of exogenously applied SA in practical agriculture have also been documented. Therefore, information regarding such recent developments needs to be compiled in the form of a book. This book aims to provide a collective information regarding SA which makes it a versatile plant growth regulator. The chapters included both theoretical and practical aspects that could be of immense use for researches and possible significant developments in future. It is intended that this book will be a help for students, teachers, and researchers, in understanding the relation between the phytohormone and agricultural sciences.