The task of Committee II of the International Commission on Radiological Protection, ICRP, is to recommend values of maximum permissible body burden of radionuclides, q, and maximum permissible concentration of these nuclides, MPC, in air and in water (or food). These values are provided only for the more important radionuclides, and they are applicable primarily to occupational exposure. This Committee has recognized that such compilations are of limited usefulness unless periodically revised to incorporate the best available information and extended to include the values required by new developments and uses. It has worked closely with several of the national committees and in particular with the Internal Dose Committee of the United States National Committee on Radiation Protection, NCRP, in collecting these data and in making revisions of the earlier publications on internal dose published by the NCRP (1953)(1) and by the ICRP (1955).(2) In addition to revising and extending the earlier publications, the members of both committees hope that this publication will be a means of harmonizing and unifying the objectives and principles used by the international committee and by the various national committees in arriving at their decisions. The hope is expressed that the national internal dose committees will apply the same basic principles of radiation protection and will adopt the permissible exposure values recommended by the ICRP or will indicate the conditions and considerations which require their modification.