Biocompatibility refers to the ability of a material to perform with an appropriate host response, in a specific application. Usage tests have been developed specifically for the evaluation of the biocompatibility of dental materials. The pulp and dentine test, the pulpotomy test and the endodontic usage test are widely recognized. The relevance, advantages and limitations of these tests are discussed with particular reference to the pulp and dentine usage test. Controversies remain in the understanding of the role of several variables in determining the outcome of this test. These are highlighted by reference to the significance of the residual dentine thickness, the smear layer and microbial microleakage at the material-cavity wall interface. Limitations in the relevance of these animal models to man due to species variations in the structure and composition of the dental tissues and their oral environment may be more perceived than real. The nature of an appropriate host response may be different in the consideration of different dental materials, for example lining materials and filling materials, and there is a need for these criteria to be reviewed.