The effects of exposure to radon on workers and members of the public have been examined for many years. Recent advances have been made in evaluating the risk associated with radon exposure and in implementing remediation programmes in dwellings. However, decisions about whether to implement countermeasures to reduce radon exposures may benefit from an enhanced capability to evaluate and understand the associated health risk. In this context, the European Commission has launched a project to develop a user friendly software package based upon current information on radon epidemiology, radon dosimetry, demography, and countermeasure efficiency. The software has been designed to perform lung cancer risk calculations specific to European populations for various exposure profiles and to evaluate, in terms of risk reduction, the efficiency of various countermeasures in dwellings. This paper presents an overview of the general structure of the software and outlines its most important modelling approaches. The software is able to evaluate the risk of fatal lung cancer associated with individual or collective exposure to radon. The individual risk calculation module determines, for an individual of given age, sex and tobacco consumption, the excess risk for a given exposure (past or future exposure) time-profile, on the basis of demographic data (WHO, 1996 & 1998) specific to the selected population. The collective risk calculation module determines the excess risk for each occupant of a dwelling, for an exposure time-profile derived from data describing the dwelling, its occupancy, and the chosen countermeasures. These calculations may be done using one of two classical approaches: 1)The epidemiological approach in which the risk is obtained directly from the radon exposure data using a risk model (BEIR IV, 1988; BEIR VI, 1999) derived from epidemiological studies of uranium miners; 2)The dosimetric approach in which the risk is derived from the radon exposure data through the successive use of a dosimetric model (NRPB, 1998) and a somatic effects model (BEIR V, 1990) based on the follow-up of lung cancers in Hiroshima and Nagasaki population. In both cases, the modifying effect of tobacco consumption on the risk is adjusted for. The principal risk indicators calculated by the software are the whole life mortality risk and loss of life expectancy. Additionally, all the intermediate results are available. A system of monetary value of human life allows also to identify the most cost-effective countermeasure. This software could play a role in general training, the provision of information to the public and in investigating the effect on risk of different dose reduction strategies. Finally, its sensitivity analysis capabilities and its database system together with its user friendly configuration capabilities should make it an easy to use tool for the risk evaluation experts of various countries to perform useful calculations, appropriate to their situations with regard to local radon and smoking patterns.