The purpose of radiation shielding is to limit radiation exposures to members of the public and employees to an acceptable level. This Report presents recommendations and technical information related to the design and installation of structural shielding for megavoltage x- and gamma-ray radiotherapy facilities. This information supersedes the recommendations in NCRP Report No. 49 (NCRP, 1976) pertaining to such medical radiotherapy facilities. Since the publication of NCRP Report No. 49, many facilities have been designed for accelerating voltages greater than the 10 MV maximum that was covered in that report. Hence recent designs have had to refer to NCRP Report No. 51 (NCRP, 1977) and NCRP Report No. 79 (NCRP, 1984) in order to account for the higher accelerating voltages and the concomitant production of neutrons. In addition, the use of barriers constructed with composite materials has become commonplace. This Report includes the necessary information for these higher accelerating voltages as well as a discussion of the various factors to be considered in the selection of appropriate shielding materials and in the calculation and evaluation of barrier thicknesses (Sections 1 through 6). Section 7 presents an extensive set of sample calculations, Appendices A and B provide supporting data figures and tables, respectively, and Appendix C discusses neutron monitoring for radiotherapy facilities. This Report is mainly intended for those individuals who specialize in radiation protection, but it will also be of interest to architects, hospital administrators, and related professionals concerned with the planning of new radiotherapy facilities.