Fretted and chaotic terrains are members of a larger family of lowland terrains on the Martian surface. They have formed in equatorial and midnorthern latitudes, within areas reasonably proximate to the extensive volcanic fields of that region. Both are relatively recent developments. Fretted terrain is characterized by extensive, smooth, lowland plains separated from the old cratered upland by an abrupt escarpment of highly irregular planimetric configuration. Chaotic terrain features jumbled assemblages of large, irregular blocks occupying lowlands or depressions within the old cratered upland. Fretted terrain is thought to evolve by recession of a steep bounding escarpment, leaving a smooth lowland floor at a remarkably uniform level. Escarpment recession is speculatively attributed to undermining by evaporation of ground ice exposed within an escarpment face, or, under a different environment, by ground water emerging at its foot. The uniform floor level may reflect the original depth of frozen ground. Removal of debris shed by the receding escarpments could be by eolian deflation, subsequent to weathering by unknown processes to produce material of uniformly small grain size, or by fluvial transport under a vastly different climatic environment. Chaotic terrain is attributed primarily to localized collapse of the cratered upland owing to removal of subsurface material, either ground ice or magma. The ground ice hypothesis suffers from possible quantitative inadequacies, but evacuation of magma seems feasible because of the extensive volcanism in the northern hemisphere. Following collapse, ground ice sapping could have been effective in causing slumps and in modifying the collapsed blocks. Eventually the floors of some areas of chaotic terrain may have been smoothed to the condition of fretted terrain, as suggested by their intimate association. These terrains reflect significant activity on the Martian surface and within the Martian interior during relatively recent times. If liquid water has been the prime agent involved in developing fretted terrain, then it is a fossil feature and carries an implication of an enduring Martian environment vastly different from the environment at present.