The various methods of monitoring worker exposure to organophosphate (OP) and carbamate (CB) pesticides were discussed. Plasma and red blood cell testing for cholinesterase was regarded as the best way to monitor exposure to OPs or CBs in the workplace. In California, where cholinesterase monitoring was mandatory, workplace evaluation and worker removal have been required if enzyme concentrations fall below certain levels. The analysis of urinary metabolites offered an alternative approach to pesticide exposure monitoring. In order to quantify exposure through urinalysis, 24 hour sampling and a knowledge of kinetics were required. In addition to determining overexposure, urinary monitoring also provided data for risk assessment, compliance, and cause of death. Assays used to determine the cholinesterase activities in the blood included the Ellman assay, radiometric assays, and field kits. Disadvantages of the cholinesterase assays, such as unsuitable storage procedures and assay conditions, led to inconsistent, often inaccurate results. The lack of standardization among reporting laboratories was considered to be a significant problem. In order to reduce the variation in analysis as much as possible, the authors recommend the immediate freezing of blood samples, the application of conversion factors to data derived from thiocholine kits, and the establishment of a standardized normal range. The authors conclude that further research is needed to improve the reliability of cholinesterase assays, thereby improving the assessment of OP and CB exposures.