Abstract: Mars exploration is an important part of human outer space activities and the human exploration of the Mars will be a next major target. The most critical issue of these missions is the safety and health of the crew. The damaging effect of ionizing space radiation is a major health concern. This paper reviews the development of Mars exploration technology in the USA and Russia. Radiation environment, radiation effect, and radiation measurements obtained onboard US spacecraft are presented and discussed. Radiation dose is predicted during the long-duration manned missions and some guidance is given for the radiation protection for future advanced manned Mars missions.