The hazard, exposure, and risks associated with asbestos fibers have been explored and debated for many years. Human evidence suggests an association between exposure to asbestos and asbestosis, lung cancer, and mesothelioma, although the lack of consistent information on fiber type, size, and exposure concentrations and duration limit our ability to establish causal relationships between exposure and disease in some cases. While uncertainties remain in our ability to consistently and accurately quantify asbestos risk to humans, progress has been made in characterizing those key factors, namely hazard and exposure, that are critical to an assessment of health risk.