RADON HAS COME TO BE RECOGNIZED as one of the most important environmental pollutants to which humans are exposed, in part due to the fact that it is widespread-indeed, radon is present in all houses-and due to the health risks associated with even average concentrations. Although the existence of radon has been known since the beginning of this century and the health effects associated with exposure to mine atmospheres (both uranium and nonuranium mines) have been studied for several decades, our understanding of it as an indoor air contaminant in ordinary houses has developed substantially only within the past decade. Some of the earliest indications of elevated concentrations in U.S. homes were associated with the use of uranium mill tailings as backfill in house construction or in other areas where radium concentrations were elevated, such as parts of central Florida, where buildings were built on lands reclaimed from phosphate mining . However, by the late 1970s, researchers had found homes in other parts of the U.S. with elevated radon concentrations for which there were no radon sources that could be associated with technological activities . The discovery of high-to-very-high indoor concentrations in eastern Pennsylvania in the mid 1980s did not offer a new scientific perspective on the radon question; rather, it focused the attention of the public and local and federal governmental agencies on the issue. This chapter provides a broad overview of radon and its radioactive decay products. A number of topics are introduced in this discussion that are covered in greater detail in later chapters.