Radon is a natural radioactive gas, which enters buildings from the ground. The joint Public Health England (PHE) –British Geological Survey (BGS) digital Indicative Atlas of radon in Great Britain presents an overview of the results of detailed mapping of radon potential, defined as the estimated percentage of homes in an area above the radon Action Level. Exposure to high concentrations increases the risk of lung cancer. PHE (formerly the Health Protection Agency (HPA)) recommends that radon levels should be reduced in homes where the annual average is at or above 200 becquerels per cubic metre (200 Bq m-3). This is termed the Action Level. Public Health England defines radon Affected Areas as those with 1% chance or more of a house having a radon concentration at or above the Action Level of 200 Bq m-3. The Indicative Atlas of radon in Great Britain presents a simplified version of the radon potential for Great Britain with each 1-km grid square being classed according to the highest radon potential found within it, so is indicative rather than definitive. The joint PHE-BGS digital radon potential for Great Britain provides the current definitive map of radon Affected Areas in Great Britain. The Indicative Atlas of radon in Great Britain is published in two documents. The area of England and Wales is published in MILES J.C.H, APPLETON J.D, REES D.M, GREEN B.M.R, ADLAM K.A.M and MYERS, A.H., 2007. Indicative Atlas of Radon in England and Wales. ISBN: 978-0-85951-608-2. 29 pp). The corresponding publication for Scotland is MILES J.C.H, APPLETON J.D, REES D.M, ADLAM K.A.M, GREEN B.M.R, and SCHEIB, C., 2011. Indicative Atlas of Radon in Scotland.).