areas of pharmaceutical safety evaluations: genetic toxicology, safety pharmacology, phototoxicity and organ toxicity. Since in vitro screening and mechanistic in vitro investigations provide early information on toxicity, the safety of patients in clinical studies has been significantly improved by the performance of in vitro studies. In addition, the high throughput of in vitro technologies allows the pharmaceutical industry to eliminate toxic structures long before 'first dose in human' studies. In vitro organ toxicity models provide important mechanistic information. However, a comprehensive analysis of the predictivity of genetic toxicity tests for rodent carcinogenicity has revealed a major problem with the specificity of the in vitro mammalian cell assays, leading to a risk that the development of efficacious drug candidates might have been stopped because of false-positive in vitro results. Therefore, data from in vitro studies, including the recently introduced human ether-a-go-gorelated gene (hERG) potassium channel inhibition test to predict QT interval prolongation and the in vitro 3T3 neutral red uptake (3T3 NRU) phototoxicity test to predict phototoxicity, should be used with great care for decision-making. All areas of in vitro safety assessments require significant refinements. Stem cell-derived cell lines and the use of 'omics' technologies are expected to make a major contribution to an improved future generation of in vitro screening assays.