Monitoring data are critical factors in an exposure assessment and thus are important elements in quantitative evaluations of hazard and risk. Monitoring is a general term used to describe a wide variety of efforts. Basically, monitoring is an activity in which specimens of selected human and environmental components are sampled and then analyzed for evidence of chemical residues indicative of potential human, animal, or plant exposure. Human and environmental monitoring programs for pesticides and selected toxic chemicals have been conducted for over 15 years. The initial ambient monitoring systems were designed to determine average concentrations of pesticides or related chemicals in human and environmental media on a nation-wide basis and determine changes in these concentrations over time.