Radon is a radioactive gas of natural origin. Its principal isotope is radon-222, stemming from uranium-238 present at various concentrations in all soils. Radon is found everywhere in the earth's atmosphere but has low reactivity by itself. Radon particulate daughters are however responsible for alpha irradiation of the bronchial epithelium. Epidemiologic studies on miners have shown long ago that radon exposure causes an increased risk of lung cancer in these workers. More recently, its risk associated with radon exposure was addressed in public dwellings. It is generally agreed that the risk of lung cancer is plausible in this setting. In particular, radon exposure in some dwellings may be comparable to that undergone by several uranium miner populations. However, simple and low-cost steps can be taken to significantly reduce radon exposure in dwellings.