Exposure to environmental toxic substances is a significant cause of human health problems. However, the critical question to ask is, at what concentrations and exposure conditions are these substances hazardous to human health. One approach to address the question is to conduct population monitoring studies using reliable biomarkers that can be used to indicate excessive exposure, identify biological damage and functional deficit, and predict increased risk for health problems. In this review, the application of traditional and functional biomarkers in population monitoring studies will be presented. A major emphasis in the review is to show how to design rigorous study protocols so that results from these studies will be appropriate for use in disease prevention programs.