In an attempt to overcome the increasing difficulty of having concise essential information on important indoor pollutants at hand, the European Collaborative Action 'Indoor Air Quality and Its Impact on Man' has published several reports amongst which a short report on radon in indoor air was published in 1988. Since then, considerable new information has become available. This report summarizes and discusses the actual state of knowledge on sources of radon in indoor air, typical concentrations, health effects and radon risk estimates, indoor radon measuring methodology, instrumentation, remedial and preventive measures to reduce indoor radon, and recommended and regulatory radon levels. The report concludes that the cost of remedial actions and the lack of public interest are major obstacles in the reduction of population exposure. Appendices which have a decay scheme of radon-222 and its short-lived progeny and which explain special quantities and units for radon and radon decay products complement the report.