1, some patients need to be orthodontic during the period of milk teeth, including the removal of bad habits, to guide the normal development of bone and subsequent permanent teeth. 2, some patients need to be orthodontic during the period of tooth replacement (9-10 years old), to guide the normal development of facial shape, correct the abnormal development of the upper and lower jaws (if the timing of treatment is delayed, facial shape and jaw development abnormalities can only be improved after adult major surgical treatment). 3, the best time for orthodontic treatment in patients at the peak of development (around 11 -13 years old), when the end of tooth replacement. 4, adult patients can undergo orthodontic treatment if they have no skeletal problems and their periodontal teeth are healthy, without age limit.