The advent of early Absorption, Distribution, Metabolism, Excretion, and Toxicity (ADMET) screening has increased the elimination rate of weak drug candidates early in the drug-discovery process, and decreased the proportion of compounds failing in clinical trials for ADMET reasons. This paper reviews the history of ADMET screening and why it has become so important in drug discovery and development. Assays that have been developed in response to specific needs, and improvements in technology that result in higher throughput and greater accuracy of prediction of human mechanisms of toxicity, are discussed; The paper concludes with the authors' forecast of new models that will better predict human efficacy and toxicity.