Radon is ubiquitous in indoor environments. Epidemiologic studies of underground miners with exposure to radon and experimental evidence have established that radon causes lung cancer. The finding that this naturally occurring carcinogen is present in the air of homes and other buildings has raised concern about the lung cancer risk to the general population from radon. I review current approaches for assessing the risk of indoor radon, emphasizing the extrapolation of the risks for miners to the general population. Although uncertainties are inherent in this risk assessment, the present evidence warrants identifying homes that have unacceptably high concentrations.