After the Viking primary mission the surface samplers and cameras continued to operate during the extended mission until early May 1978. Major extended mission accomplishments include (1) excavation of deep trenches, (2) acquisition of more samples (chiefly for the Xray fluorescence experiment), (3) construction of conical piles of materials in the sample fields, (4) backhoe touchdown experiments, (5) acquisition of contiguous pictures of the surface beneath ♯2 terminal descent engines using mirrors, (6) pushing an pulling rock, and (7) other experiments for the Physical Properties Investigation. The landing sites have continued to be monitored with the cameras during the lander continuation automatic mission, and the Lander 1 site will be monitored for a long period of time during the Viking survey mission (perhaps to December 1990 and beyond). Activities of and experiments performed by the surface samplers have disturbed the equilibrium of the surface so that wind and other processes have produced changes. Both pictures and surface sampler data acquired chiefly during the extended mission indicate that the surface materials in the sample fields of the Viking landers may be grouped into four categories (in order of increasing strength): (1) drift material, (2) crusty to cloddy material, (3) blocky material, and (4) rocks. The response of the surface materials to engine exhaust erosion combined with data from other experiments, rock populations at the sites, and theory indicates that the surface should be relatively stable and resistant to wind erosion. Although relatively stable, the erosion of the surface may occur when wind speeds are sufficiently high and when local conditions permit erosion.