Biocompatibility is one of the main prerequisites for the clinical use of biomaterials. Central to the testing of biocompatibility is the estimation of cytotoxicity, which can be assessed in vitro by using a variety of different target primary cells or cell lines. The influence of toxic agents derived from biomaterials on cellular functions and cell viability can be characterised by reductions in cell adhesion, alterations in cellular morphology, reduced cellular proliferation, and cell death, demonstrated by an absence of metabolic activity, structural disintegration and cell lysis. A brief review of the basic mechanisms of cytotoxicity and the use of different in vitro methods for testing the cytotoxicity of biomaterials is presented.