The upper atmospheric mass spectrometers flown on Viking 1 and 2 are described, and results obtained for the composition and structure of Mars' upper atmosphere are summarized. Carbon dioxide is the major constituent of the atmosphere at all heights below 180 km. The thermal structure of the upper atmosphere is complex and variable with average temperatures below 200¿K for both Viking 1 and 2. The atmosphere is mixed to heights in excess of 120 km. The isotopic composition of carbon and oxygen in the Martian atmosphere is similar to that in the terrestrial atmosphere: 15N is enrichd in Mars' atmosphere by a factor of 1.62¿0.16.