Earth-based observations and spacecraft results show that aeolian processes are currently active on Mars. Analyses of various landforms, including dunes, yardangs, and mantling sediments of probable aeolian origin, suggest that aeolian processes have been important in the geological past. Dust storms originate in specific areas of Mars and are most vigorous during the martian summer in the southern hemisphere. In order to understand aeolian processes in the low surface pressure (∼7 mb), carbon dioxide atmosphere of Mars, a special wind-tunnel was fabricated to carry out investigations of the physics of windblown particles under martian conditions. Martian threshold wind speeds have been derived for a range of particle diameters and densities; the threshold curve parallels that for Earth but is offset toward higher wind velocities by about an order of magnitude. The “optimum” size particle (the size most easily moved by minimum wind) is about 100 pm in diameter; minimum freestream winds to generate particle motion are about 40 ms-I. Grains smaller than 100 pm (“dust”) require increasingly higher winds to initiate threshold; yet, estimates of grain sizes in the dust clouds are in the size range of a few microns and smaller. Because the Viking Lander has recorded winds no stronger than those for minimum threshold, it is suggested that some other mechanism than uniform strong winds is required for “dust” threshold. Experiments and theoretical considerations suggest that such mechanisms could be cyclonic (“dust devil”) winds, a saltation cascading effect by larger (more easily moved) particles, and injection of fine grains into the wind stream by outgassing volatiles absorbed on the grains.