Children usually start to change their teeth from the age of 6 to 7, but some children’s milk teeth are always late to fall out and permanent teeth are late to erupt, why is this? We have drawn the following diagrams to explain the common causes of “retained milk teeth”. Congenital lack of permanent teeth embryo due to the lack of permanent teeth development, the root of the milk teeth can not be compressed and stimulated, does not occur resorption phenomenon, will naturally remain in the bed of the teeth for a longer period of time. Sometimes these remaining milk teeth are not lost in adulthood and still have some chewing function. Diseases of the milk teeth Periapical infection secondary to diseases of the milk teeth leads to necrosis of the permanent tooth germ; or trauma to the milk teeth damages the permanent tooth germ. Adhesion between the root of milk tooth and alveolar bone Adhesion between the root of milk tooth and alveolar bone caused by inflammation, resulting in incomplete resorption of the root of milk tooth or no resorption at all, which causes the milk tooth not to fall out. Misalignment of the permanent tooth embryo, i.e., it does not grow underneath the milk tooth, resulting in the permanent tooth not being able to make contact with the root of the milk tooth. The root of the milk tooth is not resorbed because it is not stimulated by the pressure of the permanent tooth, so it does not loosen in the dental bed, resulting in the delayed loss of milk teeth. Premature extraction of milk teeth missing teeth at the soft tissue is often friction with food so that the gingival mucosa thickening, the texture becomes tough, thus increasing the resistance of permanent teeth eruption, resulting in late eruption of permanent teeth. In addition, children’s malnutrition, endocrine metabolic disorders and other reasons and will cause children to permanent teeth late eruption.