Possibilities for harmonising controls on the radioactivity of building materials within the EuropeanUnion are being discussed in the Working Party on Natural Radiation Sources established by theArticle 31 Group of Experts (Euratom Treaty). The Working Party is preparing a document to aid theArticle 31 Expert Group and the European Commission in considering possible recommendations andtechnical guidance to the Member States for the implementation of the new Basic Safety StandardsDirective concerning the radioactivity of building materials. The discussions in the working party havedemonstrated several challenges in the possible harmonisation of controls, many of them arising fromsome significant differences in national circumstances.Small exposures from building materials are ubiquitous. Therefore, possible controls can only beapplied to exceptionally high exposures, i.e. to exposures which are feasible to control. The majorityof commonly used building materials should not be affected by any controls. A major challenge in theharmonisation is that the exposures due to building materials vary considerably between countries andareas. For example, doses in the order of 0.5 1 mSv (from external gamma radiation) might betypical for some country, where as for some other country such doses are exceptionally high and canbe feasibly restricted by using some replacing material.Divergences in national circumstances and some possible approaches for harmonising controls on theradioactivity of building materials are discussed in this paper. In addition to the dose criteria and thecorresponding investigation levels, discussed are also various other questions which need to beconsidered in the practical implementation of controls.