Orthodontics is the branch of dentistry which involves movement of the teeth through mechanical means. Initially orthodontics was taught primarily as mechanical endeavour, but the work of many researchers showed that orthodontists are dealing with the PDL and bone cells through these mechanics. Biological expression of tooth movement varies in every individual due to genetic and environmental factors. Hence understanding the cascades of events occurring after orthodontic force application helps the orthodontist to control the quantity and quality of the force system applied to the teeth. This book is an attempt to show that the biology is an integral part of orthodontic tooth movement. Understanding of these events also aids in increasing the velocity of orthodontic tooth movement while maintaining the tissue integrity. Hence the clinician's interference with the biological systems should be optimized in order to improve treatment efficacy, stability and minimize relapse.