A multiyear, planet-wide survey of Martian dust devils was conducted using observations from Mars Global Surveyor Mars Orbiter Camera, acquired through 21 January 2006. A total of 223,350 images were surveyed and 11,456 dust devils were seen in 0.4% of the images, 11.5% in the Southern and 88.5% in the Northern Hemisphere. Dust devils were observed at latitudes from 71.9¿S to 62.2¿N, over a range of surface albedos (Am~0.11--0.22) and elevations from Hellas (-8750 m) to Arsia Mons (+17,250 m). The light- and dark-toned streaks created by dust devils were observed from 80¿S to 80¿N and changed on timescales as short as 1 month. At mid-to-high latitudes, seasonal changes in streak patterns contribute to the seasonal "wave of darkening" observed telescopically from Earth. Dust devils were observed in all seasons in both hemispheres with the exception of Ls = 202.8¿--281.5¿ in the north. Peak activity occurred during midsummer in each hemisphere. Five regions in Amazonis, Syria-Claritas, Meridiani, and Gusev were monitored regularly. Amazonis had the largest dust devils and was the most active planet-wide, with annual activity occurring from Ls~8.5¿--197¿. Interannual variability resulted from dust storms and planet-encircling dust hazes, which were observed to initiate and abate dust devil activity. There was no evidence suggesting dust devils cause or lead to initiation of dust storms. Model-derived tangential wind speeds of large vortices were >20 m s-1 at 20 m above the surface. Dust flux calculations suggest that dust devils are a contributor to the background dust opacity observed through northern spring and summer.