Perseverance Valley, based on Mars Reconnaissance Orbiter HiRISE image data, is a ~180 m long, ~20 m wide set of anastomosing shallow channels superimposed on the Cape Byron rim segment of the ~22 km diameter Noachian-age Endeavour Crater on Mars. The valley cuts into the inner, eastern rim on a ~10˚ to 15˚ slope, and starts at a local low area on the rim crest. Very shallow channels, some lined with perimeter rocks, also extend from the west to meet the entrance to the valley. The western rim tilts to the west ~0.8˚ and thus these channels tilt away from the valley entrance. Several impact craters are superimposed on the valley system, indicating antiquity, although the valley’s high degree of preservation indicates that it formed after significant regional-scale fluvial erosion and diffusive smoothing of Endeavour and its rim segments. It is a unique geomorphic feature based on examination of all of Endeavour’s rim segments. The Mars Rover Opportunity has explored the western channels leading up to the entrance to the valley. As of this writing Opportunity is located on the southern side of the valley entrance, with the Athena Science Team waiting until after solar conjunction to command the rover to descend into the valley to search for geomorphic and sedimentologic evidence related to valley formation. Wind erosion along radial fractures extending into and down Cape Byron is a possibility. Debris flows are also under consideration, perhaps enabled by melting ice at the rim crest. Dry avalanches are unlikely due to the low slopes. A fluvial origin is a strong contender based on models that show it is possible to have had a western catchment present when the Burns formation hydrated sulfates were being emplaced, followed by self-compaction of these sediments that tilted the western plains away from the rim crest. The key to testing among the various hypotheses for formation of the valley system will be the detailed stereo and multispectral imaging observations Opportunity will make of morphology and deposits at ~20 m intervals during its descent along the valley floor. Opportunity is in fact conducting the first ground-based exploration of a candidate fluvial valley system on Mars, and data from the rover will provide unique information on formation processes, including the role of water in shaping valley landforms.