As NASA considers the possibility of sending manned spaceflight missions to Mars, radiation protection for crew members remains one of the key technological issues which must be resolved. Before this problem can be solved, it is important to understand and characterize the radiation environment not only near Mars and on its surface but also in the interplanetary space between the Earth and Mars. The Martian Radiation Environment Experiment (MARIE) was designed for that purpose. It is a payload on the 2001 Mars Odyssey Orbiter, which was launched on April 7, 2001 and was captured in Mars orbit on October 24, 2001. The instrument measures doses from two sources of space radiation which are harmful to humans — galactic cosmic rays (GCR) and solar energetic particles (SEP). MARIE provided data during the curise from the Earth to Mars, and it continues to provide data now that the spacecraft is in orbit at Mars.