Teeth yellow light gray or dark gray is the clinical manifestation of tetracycline teeth, teeth yellow light gray or dark gray, generally the front teeth than the back teeth milk teeth than permanent teeth staining more obvious. Heavy cases have enamel hypoplasia. There are many reasons for tooth staining and yellowing, which can be broadly divided into two aspects, namely endogenous and exogenous. Exogenous staining is due to the presence of a variety of bacteria on the surface of the teeth, they secrete many sticky substances, daily dietary tea scale, tobacco stains, and certain minerals in the drinking water adsorbed on these sticky substances, and gradually make the teeth yellow or black. Symptoms are: yellow, in the sunlight is bright yellow fluorescence, and later gradually from yellow to brown or dark gray. This transformation is slow and can be facilitated by sunlight, so the labial surfaces of the incisors are the first to change color. Anterior teeth are more obviously colored than posterior teeth; the coloration of milk teeth is more obvious than permanent teeth, because the enamel of milk teeth is thinner and more transparent, and it is not easy to cover the color of tetracycline conjugates in dentin. The degree of tooth coloration is related to the type of tetracycline, the dose, and the number of doses administered. It is generally accepted that shrinkage tetracycline, norethindrone, and tetracycline hydrochloride cause more pronounced staining than do oxytetracycline and aureomycin. In permanent teeth, the number of courses of tetracycline is positively related to the degree of coloration, but a large dose given over a short period of time has a greater effect than an equal total dose given over a long period of time. It is important to note the differential diagnosis with the following symptoms: Brownish-black teeth: Abnormal change in the color of the teeth to a brownish-black color. Intra-teeth staining is tooth discoloration caused by alterations in the tissue structure or nutrition of the tooth, commonly associated with pulp necrosis, mottled enamel and discoloration of the tooth due to drug deposition. Extrinsic tooth coloring is pigmentation on the surface of the tooth. Teeth yellowing: due to a variety of different reasons, the teeth appear yellow Teeth color abnormality: because the surface of the teeth covered with a layer of enamel, transparent or translucent state, and its deep for dentin, yellowish. The color of the teeth is related to the degree of calcification of the enamel, the higher the degree of calcification, the more transparent the enamel, and the deeper the dentin of the original color through which the teeth are yellowish, the color of the teeth is abnormal in a variety of dental diseases.