The cavities formed by dental caries are categorized into 5 types according to the site of caries; 3 types according to the number of surfaces involved. 1. The site of caries: Class I cavities are cavities prepared by caries in points, gaps and grooves of all teeth; Class II cavities are cavities prepared by caries in the neighboring surfaces of the posterior teeth; Class III cavities are cavities prepared by caries in the neighboring surfaces of the anterior teeth that do not involve incisal angles; Class IV cavities are cavities prepared by caries in the neighboring surfaces of the anterior teeth that involve incisal angles; and Class V cavities are cavities prepared by caries in the labial, buccal, and lingual cervical 1/3 of all teeth. 2. Number of surfaces involved: caries involving one surface is called a single-sided cavity; involving two surfaces is called a double-sided cavity; involving more than two surfaces is called a complex cavity. The more complex the cavity is, the more difficult it is to fill and treat it. The composite resin, which is the commonly used filling material, requires high moisture barrier and drying conditions, so it is recommended to remove as much as possible of the decayed tissue and use a rubber barrier.