Since the identification in the 1950s of deoxyribonucleic acid as the building block of life, the impact of molecular biology has been far-reaching. Understanding the processes of how DNA is replicated, transcribed into RNA and then translated into protein products has not only provided a fundamental knowledge of life but has also spawned a plethora of applications. Molecular biology has been high profile and widespread in research into the biology of disease and in drug discovery. It has additionally found application in understanding the adverse effects, or toxicity, of candidate drugs and how they interfere with biochemical and biological processes. In recent times the biggest impact of molecular biology in toxicology has been through the study of differential gene expression, largely as a result of the advent of genomics. This review seeks to describe how toxicogenomics strategies have been implemented and integrated into nonclinical studies of drug safety.