At NASA's request, the National Council on Radiation Protection and Measurements (NCRP) was asked to consider radiation exposure to aviation crews and passengers on commercial flights resulting from high altitude flights. The report reviews current knowledge about radiation exposure and risks and their uncertain application to higher altitude flights. Recommendations for further studies and research are presented. Background and relevant considerations included dose rates at different altitudes, dose rates for solar flares, biological effects of ionizing radiation, and limitations on current risk estimates. The discussion of estimates of risk to crew and passengers examines the flight crew, frequent travelers, conventional travelers, cumulative risk, and prenatal exposure. A radiation protection philosophy is presented.