Impaired eruption of teeth, which is usually seen clinically in permanent teeth, is mostly caused by retention of milk teeth. Premature loss of milk teeth or premature extraction may also cause difficulty in the eruption of permanent teeth. The normal eruption time of teeth may vary from one individual to another, and it is difficult to determine the limit of tooth eruption time for a particular individual clinically. For practical clinical purposes, a time range can be determined based on the average age of normal eruption, and once this range is exceeded, the eruption can be considered abnormal. The clinical estimation of the time of tooth eruption is also based on the time of initial eruption of the individual (first tooth eruption) and the time range of the individual’s tooth eruption. The order of eruption of the milk teeth is constant, while the order of eruption of the permanent teeth just varies individually, mainly between the upper cuspids and the first and second bicuspids. In clinical practice, parents often come to consult about the eruption of their children’s teeth. In order to provide parents with a better understanding of the relevant knowledge, the common dental eruption abnormalities and the principles of treatment are introduced as follows: Early eruption: Early eruption of milk teeth is less common than late eruption. Some infants have erupted teeth at birth or soon after birth, and these teeth are mostly in the position of the incisors in the lower jaw. They are usually one tooth, but occasionally two teeth may erupt. Most of these teeth are normal milk teeth, but they may also be extra teeth. The teeth that erupt at birth are called ‘birth teeth’, ‘birth teeth’ or ‘delivery teeth’. Teeth that emerge soon after birth are called “newborn teeth” or “neonatal teeth”, which are defined by time and do not imply any morphological differences or clinical characteristics. Some survey data show that the incidence of early eruption of milk teeth is about 0.05%, with a higher incidence in female infants than in male infants. The reasons for the early eruption of “birth teeth” and “newborn teeth” are not well understood, one theory is that the teeth erupt too early because the germ is too close to the oral mucosa, and some believe that it may be related to family heredity. Most of these teeth have no or little root development because they are still in the early stages of development, and these teeth are usually loose and mostly attached to the mucous membrane at the top of the alveolus, thus risking falling out and being inhaled by the baby. Therefore, regardless of whether the early eruption is normal or multiple teeth, if they are extremely loose and have the possibility of falling out, they should be extracted promptly. However, when such teeth are not loose, or not very loose, and not easy to fall off, but because of early eruption, affect the baby sucking, can not breastfeed or sucking teeth rubbing the tongue ligament and its two sides of the mucosa caused by decubitus ulcers, long-term non-healing, then should stop sucking breastfeeding, switch to spoon feeding breast milk, in order to avoid teeth rubbing. When ulcers have been caused, gentian violet can be applied to the ulcer surface to promote healing, with good results. Some people advocate slightly sharpening the cutting edge of the teeth to reduce friction, but the sharpened cutting edge is often rough and it is impossible to grind away too much of the cutting edge of the teeth, so the effect is not ideal. When this decubitus ulcer, which sometimes has turned into a chronic proliferative change, is easily mistaken for a tumor and taken for biopsy or excision. Difficulty in eruption: If the mastoid incisor is lost too early, the child is accustomed to chewing with the gums, making the gums more keratinized, tougher or even thicker, resulting in difficulty in eruption of permanent teeth. At this time, we can find localized gums that are pale and raised, and when we touch them, we can touch the hard cutting edge of the teeth below the gums. In this case, if the tooth does not erupt after a long delay, the gingival piece at the incisal edge can be cut away to expose the incisal end of the tooth, which will speed up the eruption of the tooth. It is rare that the gingiva needs to be cut to help the tooth erupt. Sometimes a localized hematoma is produced on the molar surface that is about to erupt or on the incisive margin of the incisor, which is swollen and protruding, with a blue-brown color and a wavering sensation when palpated. After cutting and releasing the blood, and then cutting away the local gum piece, the teeth can erupt normally. Retained milk teeth: permanent teeth have erupted, but the corresponding location of the milk teeth have not yet fallen out, called retained milk teeth. When individual milk teeth are over the age of tooth replacement and do not fall off for a long time, and the permanent teeth in the corresponding position have not erupted, it is also called retention of milk teeth, which is mostly due to the blockage of permanent teeth or congenital absence. If the permanent teeth are congenitally missing and the retained milk teeth are not loose or diseased, the milk teeth can be retained. If the root development and embryonic position of the permanent teeth are normal, the retained milk teeth can be extracted and the permanent teeth can erupt on their own. Sometimes the retained teeth are multiple teeth and should be extracted if they affect the normal alignment of permanent teeth. Late eruption of teeth: Usually the first milk tooth erupts around the age of one week, which can be considered normal. If the first milk tooth does not grow for a long time beyond one week of age, it should be considered whether there are any systemic effects or disorders, such as rickets, cretinism (stupid dwarf disease), extreme nutritional deficiency, etc. Children with congenital syphilis may have late eruption of teeth (but also early eruption). Therefore, children with late tooth eruption should be examined for systemic factors, and it is inappropriate to take nutritional or endocrine therapy to promote tooth eruption without identifying the cause. Clinical attention should also be paid to the possibility of “edentulous malocclusion” in children who do not grow their first milk teeth for a long time, which should be identified by x-ray. Ectopic eruption: Any permanent tooth that deviates from its normal position during eruption, or permanent tooth that does not erupt in its normal position in the dental row, is called ectopic eruption of teeth. Ectopic eruption often causes abnormal resorption of the adjacent milk teeth. Ectopic eruption mostly occurs in the maxillary first permanent molar, followed by the mandibular lateral incisor and the mandibular first permanent molar. Sometimes it can also occur in other teeth. When this condition is detected, a pediatric dentist should be consulted for a clear diagnosis and proper treatment measures.