This Report is the report of a joint task group of the ICRU and the International Commission on Radiological Protection to the two Commissions. It represents the culmination of an effort that the Commissions initiated when radiobiological findings indicated that protection recommendations for some high-LET radiations might not offer the same margin of safety as those for low-LET radiations. Sections of the report cover conceptual approaches to radiation protection C the limitation approach and the assessment approach; specification of radiation quality C in terms of linear energy transfer and lineal energy; and relative biological effectiveness (RBE) C autonomous and non-autonomous responses of cells and experimental findings. Brief appendices treat quantitative relations in radiation protection systems, the relationship between distributions in linear energy transfer and lineal energy, and theoretical considerations on RBE. Another appendix includes a comprehensive evaluation of the experimental data on RBE of high- vs. low-LET radiation.