NASA's Mars Geodesy/Cartography Working Group (MGCWG), established in 1998 and chaired since 2000 by one of us (TCD), consists of leading researchers in planetary geodesy and cartography at such diverse institutions as JPL, NASA Ames and Goddard Centers, Purdue and Ohio State Universities, Malin Space Science Systems, the German Center for Aerospace Research DLR, and the US Geological Survey, as well as representatives of the current and future Mars mission teams that are the customers for Mars maps. The purpose of the group is the coordinate the activities of the many agencies active in Mars geodesy and cartography in order to minimize redundant effort and ensure that the products needed by mission customers are generated. A specific objectives has been to avoid repeating the experience of the 1970s-80s, when competing researchers produced geodetic control solutions and maps of Mars that were mutually inconsistent. To this end, the MGCWG has recently assembled a set of preferred values for Mars cartographic constants, based on the best available data. These values have been transmitted to the International Astronomical Union and appear in the report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites as the officially recommended constants for Mars (Seidelmann et al., 2002). The MGCWG has also recommended to NASA that the USGS adopt the IAU-approved coordinate system of planetocentric latitude and east longitude for future maps of Mars, in the place of the (also IAU-approved) planetographic system with west longitude positive. This recommendation has recently been approved by NASA. In this paper we present the preferred values for Mars cartographic constants with discussion of the process by which they were derived, then discuss the rationale and implications of the use of east/planetocentric coordinates in future Mars maps.