Nonclinical safety assessment is an essential component of drug discovery and development. Early in the drug discovery/development continuum, there are no regulatory guidelines and sponsors typically harness a range of in silico, in vitro, and short-term in vivo toxicology screens and pilot studies to develop comparative toxicity profiles for compounds under consideration. Results from these early evaluations can be used to terminate the development of compounds with undesirable toxicity liabilities as well as to prioritize other compounds for further evaluation. As drug candidate molecules approach entry into clinical trials, regulatory expectations regarding nonclinical safety assessment become more explicit. Scientists in the pharmaceutical industry have worked with their regulatory counterparts to establish an organizational structure – the International Conference on Harmonisation (ICH) – to develop internationally accepted guidelines that specify requirements for nonclinical safety assessment activities in a phase-specific manner. The ICH guidelines cover the need for repeat-dose toxicology studies of increasing duration in rodents and nonrodents, safety pharmacology studies, genetic toxicity tests, immunotoxicity and immunogenicity evaluations, phototoxicity assessments, developmental and reproductive toxicity studies, and carcinogenicity assessments. In planning, conducting, and reporting these studies, the toxicologist must ensure that a fully integrated assessment of potential safety liabilities associated with the test drug is communicated to the development team so that potential safety concerns can be properly addressed.