This report evaluates the risks to human health and the environment posed by thallium, a naturally occurring, ubiquitous heavy metal present at low levels in drinking-water, food, and ambient air. Although the industrial production and uses of thallium are now limited, thallium(I) sulfate was previously used as a therpeutic agent to reduce sweating and to cure various infections, including tuberculosis and malaria, and as a depilatory agent, resulting in numerous cases of poisoning. A review of the methods for the determination of thallium in biological and environmental samples is followed by an assessment of sources of human and environmental exposures.