Epidemiologists are now embarking on the evaluation of the hypothesis that exposure to radio frequency energy from low-power wireless communication devices, such as portable cellular telephones, causes brain cancer and other adverse health outcomes. Even in the laboratory, exposures from radio frequency sources are difficult to quantify; their measurement in large populations for epidemiologic study is challenging. In this paper, we outline the nature and magnitude of these exposures and discuss the prospects for obtaining useful measures of exposure for epidemiologic research.