In the first of three sections of this book, an overview of in vitro toxicology is provided. The molecular biology of cellular toxicity, receptor-mediated mechanisms, structure-activity relationships, pharmacokinetic modeling, and the use of biosensors in toxicological studies are treated in detail. The second section covers some of the very important aspects that allow the analysis of the mechanisms of toxicity, e.g. intercellular communication and signal processing, the cytoskeleton, apoptosis, and free radical mediated toxicity. The last section presents in vitro studies on hepatotoxicity, dermal toxicity, immunotoxicity, nephrotoxicity, and respiratory toxicity.