While the effects of acute high-dose irradiation are well-documented, less is known about the effects of low level chronic radiation exposure. Physical dosimetry cannot always be relied upon, so dose estimates and determination of past radiation exposure must often be based upon biological indicators. Some of the established methods used in the assessment of nuclear accidents are reviewed here, including cytogenetic analyses, mutation-based assays and electron spin resonance. As interest in research on low-level radiation exposures expands, there is an increasing need for new biomarkers that can identify exposed individuals in human populations. Developments in high-throughput gene expression profiling may enable future development of a rapid and noninvasive testing method for application to potentially exposed populations.