Each tooth is attached to the alveolar bone by the periodontal ligament (PDL). The major components of PDL are the principle collagenous fibers organized in bundels. Except for fibers, the ligament consists of four types of cellular elements. Alveolar bone contains 1/3 organic matrix (collagen and non – collagen proteins) and 2/3 anorganic components (calcium, phosphates, carbonates, cytrates..) During the masticatory function the incompressible tissue fluid within the PDL space prevents quick displacement of the tooth. Instead of this, the force is transmitted on alveolar bone which bends and produces piezoelectric signal as an important stimulus to skeletal regeneration. The orthodontic tooth movement relates with blood flow theory and relies on chemical changes. Removable and fixed appliances produce forces that cause tooth movement. There are several types of movement. Initial tissue reaction on the force appears within several minutes. Compression of the PDL may cause ishemic and hyalinized zone. Then the tooth movement is stopped until the hyalinized zone is eliminated by undermining resorption. Deleterious effects of orthodontic force are root resorption, pain and pathologic tooth movement. The retention period continues to the active period of treatment. It should retain achieved treatment effect and prevent relapse.