The need for biocompatible materials implies the necessity of toxicity testing. The toxicity of a dental material can be evaluated by in vitro tests, by animal experimentation including usage tests, and by clinical studies in humans. In vitro studies are mainly performed to evaluate the cytotoxicity (cell damage) or the genotoxicity (specific DNA damage or chromosomal aberration) of a dental material. Cell or tissue cultures play an important role because they allow a maximum standardization. This paper describes their application and interpretation of results in biocompatibility tests.