Martian surface materials viewed by the two Viking landers (VL-1 and VL-2) range from fine-grained nearly cohesionless soils to rocks. Footpad 2 of VL-1, which landed at 2.30 m/s, penetrated 16.5 cm into very fine grained dunelike drift material; footpad 3 rests on a rocky soil which it penetrated ?3.6 cm. Further penetration by footpad 2 may have been arrested by a hard substrate. Penetration by footpad 3 is less than would be expected for a typical lunar regolith. During landing, retroengine exhausts eroded the surface and propelled grains and rocks which produced craters on impact with the surface. Trenches excavated in drift material by the sampler have steep walls with up to 6 cm of relief. Incipient failure of the walls and failures at the end of the trenches are compatible with a cohesion near 10--102N/m2. Trenching in rocky soil excavatd clods and possibly rocks. In two of five samples, commanded sampler extensions were not achieved, a situation indicating that buried rocks or local areas with large cohesions (?10kN/m2) or both are present. Footpad 2 of VL-2, which landed at a velocity between 1.95 and 2.34m/s, is partly on a rock, and footpad 3 appears to have struck one; penetration and leg strokes are small. Retroengine exhausts produced more erosion than occurred for VL-1 owing to increased thrust levels just before touchdown. Deformations of the soil by sampler extensions range from doming of the surface without visible fracturing to doming accompanied by fracturing and the production of angular clods. Although rocks larger than 3.0 cm are abundant at VL-1 and VL-2, repeated attempts to collect rocks 0.2--1.2 cm across inbedded in soil indicate that rocks in this size range are scarce. There is no evidence that the surface sampler of VL-2, while it was pushing and nudging rocks ?25 cm across, spalled, chipped, or fractured the rocks. Preliminary analyses of surface sampler motor currents (?25 N force resolution) during normal sampling are consistent with cohesionless frictional soils (ϕ?36¿) or weakly cohesive frictionless soils (C<2kN/m2). The soil of Mars has both cohesion and friction.