Prior to the Mars Global Surveyor (MGS) mission, a very general view had emerged in which Martian surface materials were seen as consisting of a mixture of bright dust, dark sand, and rocks. The configuration of these materials and the winds that transport and deposit sand and dust have been thought to be directly linked to the albedo patterns that have been observed on Mars for centuries. High spatial resolution images (1.4--20 m/pixel) obtained by the MGS Mars Orbiter Camera (MOC) between September 15, 1997, and July 4, 1999, provide new information about the physical nature of the windblown material on the Martian surface and show that the pre-MGS view was much too simple. In addition to bright dust and dark sand, MOC images show evidence of bright sediment that can be transported by saltation (e.g., sand) and dark material that can be transported in suspension (e.g., silt). New views of eolian wind streaks in Daedalia Planum show that part of this region, thought to be mantled by bright dust based upon Viking and Mariner observations, is instead covered by a thin veneer of bright, windblown sand. MOC images of Sinus Sabaeus and parts of Syrtis Major, two regions thought to be sandy based upon Viking era observations, exhibit thick mantles that are inferred to consist of fine-grained sediment deposited from eolian suspension. Low albedo wind streaks in western Arabia Terra are also dark mantles, and their association with eroded crater floors and megaripples/dunes on these floors suggest that these particular wind streaks are deposits of silt-sized material that was only briefly suspended before settling to the surface. MOC images also show evidence that some eolian dunes are active on Mars today (i.e., in 1998 and 1999); the evidence for activity is largely based upon wind- and avalanche-induced streaks superposed on or eroded into seasonal frost on high-latitude dune fields. MOC images show that some other dunes are inactive, but the albedo of dunes relative to surrounding terrain is not a good indicator of dune activity because some inactive dunes are not mantled by dust. Inactive dunes are best identified by superposed features such as impact craters, landslide deposits, and yardangs. ¿ 2000 American Geophysical Union