Radon is a radioactive gas that emanates from uranium-bearing soil and porous rock. Although radon is most highly concentrated in areas of high uranium concentration, the presence of trace amounts of uranium in most ground sources means that all humans are exposed to radon to some degree. Radon migrates out of soil and rock into the surrounding air, resulting in accumulation in poorly ventilated or closed areas. Such areas represent the primary environments in which humans are exposed to radioactivity from radon to experience detrimental health effects. There is no convincing evidence that any cancers other than lung cancer are associated with exposure to radon. There is, on the other hand, consistent evidence of a substantially elevated risk of lung cancer among Canadians exposed to radon in certain occupational settings, particularly uranium mining. While the combined evidence for a positive association between residential radon exposure and lung cancer is less compelling, the inherent methodological difficulties in mounting such studies may render it impossible for any single study to detect the relationship more conclusively. The best available evidence to date from pooled analyses indicates a positive, but weak association between residential radon and lung cancer risk. Residential radon is of critical importance because it is ubiquitous; a small excess risk that may exist in relation to radon exposures encountered in a residential setting translates into the potential for a far greater number of excess cancers in the general population than does exposure of a relatively small number of miners, even though the latter may be exposed to much higher levels of ionizing radiation. Fortunately, a number of techniques are available to homeowners to reduce radon concentrations in their homes.