This review covers current developments in molecular-based studies of the structure and function of carboxylesterases. To allay the confusion of the classic classification of carboxylesterase isozymes, we propose a novel nomenclature and classification of mammalian carboxylesterases on the basis of molecular properties. Mechanisms of the regulation of the gene expression of carboxylesterases by xenobiotics, and the involvement of carboxylesterase in drug metabolism are also described. The novel biomarker for organophosphate pesticide exposure developed here is much more useful and reliable than cholinesterase inhibition.