Compatibility between living tissues and implanted materials may be impossible. An implant may be tolerated, however, for varying periods of time and a dynamic equilibrium between host and implant may be established. The effects of the implant on the host may be: acute toxicity, chronic toxicity, allergenicity, cancerogenicity, teratogenicity. The host environment may cause a breakdown of the implant (chemical or mechanical), and the breakdown products may again interact with the host. Biomaterials Science is a descriptive science; biomaterials research has produced various types of classifications. With further research it may progress to an explanatory science. A quantitative assessment of the tolerance of an implant, which is necessary as a basis for the formulation of standards and for the design of predictive tests, may not be possible until we can explain the dynamic equilibrium between host and implant.