Assessments of doses and risks to workers and members of the public exposed to radionuclides require biokinetic and dosimetric models which describe the behaviour of the radionuclides and provide tools for dose calculations. The overall objective of BIODOS is to improve the scientific basis of existing models and to provide new or improved models. One part aims to provide models targeting specific uncertainties, including estimates of doses and risks from radionuclides ingested by adults and children; estimates of the transfer of radionuclides to breast milk and doses to infants; development of systemic models to improve the interpretation of bioassays; and studies of the importance of heterogeneous distribution of dose within tissues and cells. The other part forms a comprehensive research programme on the inhalation of radionuclides and addresses four areas of uncertainty, namely ultrafine particles, inter-subject variation, absorption into blood and clearance mechanisms.