Abstract :Orthodontic tooth movement depends on favorable supporting tissues responding to the forces used during treatment. The tissues involved are the periodontal ligament (PDL), alveolar bone, cementum and dentin. Clast cells play an important role in the process of physiologic remodeling and mineralized tissue responses to external forces. They also play a significant role in unfavorable iatrogenically induced root resorption. Understanding the tissues and cells involved is essential for modulating the biology of orthodontic tooth movement and the prevention of the unwanted side effects of orthodontic treatment.