Book excerpt: With the recent rapid developments in drug discovery, including advances in both drug design and combinatorial chemistry, toxicology is becoming an increasingly important proactive tool. In addition to new compound testing, toxicity studies can also shed light on normal bodily processes through the understanding of adverse ones. Molecular Toxicology is a concise introduction to the subject, taking the reader through the theoretical principles of toxicology followed by specific examples. In the first section, the concepts behind possible mechanisms of toxicity are described (e. g. the specific enzyme or receptor system) using examples where appropriate. Following this a series of examples are used to show the extension of concept into the real world, in an organ specific manner. The book concludes with a section outlining toxicity assessment methods, where the impact of molecular biology is having a considerable impact, including DNA microarrays, proteomics and bioinformatics.