The biocompatibility of a dental material depends on the compositions of the material and the location and interactions of the material in the oral cavity. Metal ceramic and polymer materials all elicit different biological responses, because of the indifference in composition. Furthermore, diverse biological responses to these to these materials depend on whether they release their components and if those components are toxic, immunogenic or mutagenic at the released concentrations. The location of a material in the oral cavity also partially determines its biocompatibility. Materials that appear biocompatible when in contact with the oral mucosal surfaces may cause adverse reactions if they are implanted beneath it. Materials that are toxic when in direct contact with pulp may be essentially innocuous if placed on dentin or enamel. Finally interaction between the material and the body influence the biocompatibility of the material. The material’s response to changes in pH, the application of force, or the degenerative effects of the biological fluids can alter its biocompatibility. Features of a material’s surface that promote or discourage the attachment of bacteria, host cells or biological molecules determine whether the material will promo the plaque retention, integrates with bone or adhere to dentin.