Restorative dentistry is the process of filling or repairing teeth. This is most often done when dental decay, or a cavity, forms on the tooth. There are three main layers of tooth material: the enamel, dentin, and pulp. Cavities contained to the robust enamel are most often monitored closely and treated with remineralization techniques. Once a cavity has progressed to the softer dentin layer, a filling is often necessary. The decayed tooth structure is removed and replaced with either a tooth coloured composite or a silver amalgam restoration. If a cavity grows too large, it can affect the pulp, or nerve, of the tooth. When a cavity reaches this stage, often root canal, or endodontic, therapy is necessary.