Massed crescentic ridges are the most common dunes in the north circumpolar erg and crater floor dune fields on Mars; they are similar in plan to dunes that are typical of many desert basin ergs and dune fields on Earth. This correspondence implies that the dynamics of dune formation are similar on the two planets, despite martian constraints on dune formation that include much higher velocity winds required to move 'sand' in saltation, the possible inhibition of sand movement by asbsorbed water vapor and the seasonal 'snow' cover in the north circumpolar erg, and a probably sparse sand supply. Scale ratios derived from measurements of martian dunes at latitude 72 ¿N, longitude 53 ¿ and latitude 47.5 ¿S, longitude 331 ¿ are almost identical to scale ratios for much smaller gypsum duens at White Sands, New Mexico. Isolated, scattered barchan dunes probably show thinning of dune sediments toward the southern margin of the north circumpolar erg on Mars, indicating a diminished sand supply and/or deflation of parts of the region by high speed winds. A complex dune formed by the apparent collision of barchans on Mars suggests that dunes of different sizes there, as on Earth, migrate at different rates. The present activity of the martian dunes, however, is not known, and they may be at least seasonally inactive. Uniform dune patterns in areas of 'sand' accumulation on Mars, as on Earth, reflect the long-term adjustment of eolian bedforms to wind regimes, sand supply, and topographic variations. Boundary ridges of sand that delineate the margins of many topographically controlled dune fields and ergs on Earth also occur on Mars. Complex akl¿ patterns on Mars, as on Earth, probably reflect locally complex wind regimes. Wind erosion grooves in some of the martian dunes, and the apparent superposition of some dune patterns in the north circumpolar erg may record changes in the eolian regime there. Preservation of these features implies considerable cohesion of the dune sands. Large through-going longitudinal dune belts, common on Earth, are not seen on Mars. The absence of 'sand-passing' dunes of this type and the restriction of 'sand-trapping,' massive crescentic dunes to a few sites on Mars, suggest that much of the available sand has been removed from the martian plains. Mars may have a long eolian history in which much of the sand suitable for saltation has already been transported to the north polar erg and crater floor dields. On Earth the formation of 'mature' ergs is estimated to require 2¿106 years or more. By analogy, the massive dunes on Mars probably required as much or more time to accumulate. The age of their formation is not known; it may extend to periods when climatic conditions were possibly more earthlike than at present.