Viking orbiter observations extending over 1 Martian year have been used in conjunction with Mariner 9 data obtained in 1971–1972 to study the seasonal and secular behavior of several kinds of wind streaks. Most bright streaks, inferred to consist of dust storm fallout in the lees of obstacles, have changed very little in form or orientation over a period of 3 Martian years. Some are extremely stable and have experienced no effective eolian action over the 3 years. A few bright streaks changed rapidly during global dust storms; these streaks are located in areas subject to both global and topographic winds. Viking images have shown for the first time that dark, erosional streaks are stable from the time of their formation after major dust storms until the onset of the next episode of major storm activity. Available evidence shows that the large, dark streaks in Oxia Palus consist of material deflated from dune fields within the associated craters. These streaks lengthened secularly since 1972; changes appear to occur episodically during southern summer. The great majority of all streaks reflect winds during the period from late southern spring to early southern fall, although some changes occur throughout the year. The global pattern of wind streaks and the variability of the streaks thus depend strongly upon the current south-north asymmetry of seasons on Mars.