The Valles Marineris, an enormous canyon system spanning more than one quarter of the equatorial girth of Mars, exhibits in its landforms the consequences of uniquely Martian extensional tectonics and a variety of erosional and depositional processes. Reported here are new insights into the evolution of the canyon system and possible evidence for cyclical climate change from the equatorial region. Tectonic control appears to be the fundamental influence on canyon form and evolution, but the style or intensity of tectonism appears to be regionally variable. In the region of the west and central canyons, chains of elongate pits, graben, and the actual troughs are all inferred to be manifestations of pronounced north-south and secondary east-west crustal extension. It is proposed that this region of the Valles Marineris is made up of a large number of discrete elongate blocks which have shifted vertically and tilted in relation to one another. Depending on the geometry of this adjustment the surface layer subsides as a coherent block of collapses to form a chain of pits. In the eastern canyons and chaotic terrain the tectonics appear to follow a different pattern, as the crustal layer breaks up into large patches of equant blocks, seemingly reflecting a lesser amount of crustal extension. Diverse wall features and varied landslide morphologies in the canyons indicate that material has been transported from the walls of the troughs to the floors, contributing to their widening. The paucity of small impact craters inside the canyons suggests that this process spans a large part of Martian history, perhaps even up to the present day. Small, sharply defined scarps cut erosional features on many canyon walls, suggesting that the driving force for canyon enlargement continues to be downfaulting. This conclusion is supported by the absence of evidence for excavation by exogenic process. Many troughs are segmented into closed basins, precluding lateral transport of debris except by acolian deflation, for which there is little evidence. Substantial accumulations of regularly layered sediments have been recognized on the floors of at least two unconnected canyons. Cyclical variations in sedimentation rates or conditions are implied. The stratigraphic relationships among various layered and unlayered canyon interior materials as well as dune fields require that substantial deposition and reerosion have taken place within some canyons since their formation. The pervasive influence of extensional tectonics in the Valles Marineris leads us to doubt that the decay of ground ice or volcanism needs to be invoked to explain collapse in the eastern canyons or chaotic terrain. We find no persuasive evidence for former fluvial episodes in the trough system proper, where most erosional features can be accounted for by mass wasting. There are suggestions, however, that cyclical climate change has played a role in the development of layered materials in the canyons in the same way that it has apparently controlled layering of deposits in the polar regions. Prolonged tectonic activity in the Valles Marineris, on the east flank of the Tharsis-Syria Rise, may correlate with prolonged volcanism and tectonism in evidence on the west flank. The long-lived dynamic process in the deep Martian interior suggested by this relationship may also be responsible for large gravity anomalies associated with the rise.