Introduction The Mars Global Digital Dune Database presents data and describes the methodology used in creating the database. The database provides a comprehensive and quantitative view of the geographic distribution of moderate- to large-size dune fields from 65? N to 65? S latitude and encompasses ~ 550 dune fields. The database will be expanded to cover the entire planet in later versions. Although we have attempted to include all dune fields between 65? N and 65? S, some have likely been excluded for two reasons: 1) incomplete THEMIS IR (daytime) coverage may have caused us to exclude some moderate- to large-size dune fields or 2) resolution of THEMIS IR coverage (100m/pixel) certainly caused us to exclude smaller dune fields. The smallest dune fields in the database are ~ 1 km2 in area. While the moderate to large dune fields are likely to constitute the largest compilation of sediment on the planet, smaller stores of sediment of dunes are likely to be found elsewhere via higher resolution data. Thus, it should be noted that our database excludes all small dune fields and some moderate to large dune fields as well. Therefore the absence of mapped dune fields does not mean that such dune fields do not exist and is not intended to imply a lack of saltating sand in other areas. Where availability and quality of THEMIS visible (VIS) or Mars Orbiter Camera narrow angle (MOC NA) images allowed, we classifed dunes and included dune slipface measurements, which were derived from gross dune morphology and represent the prevailing wind direction at the last time of significant dune modification. For dunes located within craters, the azimuth from crater centroid to dune field centroid was calculated. Output from a general circulation model (GCM) is also included. In addition to polygons locating dune fields, the database includes over 1800 selected Thermal Emission Imaging System (THEMIS) infrared (IR), THEMIS visible (VIS) and Mars Orbiter Camera Narrow Angle (MOC NA) images that were used to build the database. The database is presented in a variety of formats. It is presented as a series of ArcReader projects which can be opened using the free ArcReader software. The latest version of ArcReader can be downloaded at http://www.esri.com/software/arcgis/arcreader/download.html. The database is also presented in ArcMap projects. The ArcMap projects allow fuller use of the data, but require ESRI ArcMap? software. Multiple projects were required to accommodate the large number of images needed. A fuller description of the projects can be found in the Dunes_ReadMe file and the ReadMe_GIS file in the Documentation folder. For users who prefer to create their own projects, the data is available in ESRI shapefile and geodatabase formats, as well as the open Geographic Markup Language (GML) format. A printable map of the dunes and craters in the database is available as a Portable Document Format (PDF) document. The map is also included as a JPEG file. ReadMe files are available in PDF and ASCII (.txt) files. Tables are available in both Excel (.xls) and ASCII formats.