Introduction: The MSL APXS was selected by NASA as a Canadian contribution for the Mars Science Laboratory, now scheduled for launch in 2011. It follows the Pathfinder and Mars Exploration Rover APXS instruments [1] and was significantly improved in many ways. The APXS in-situ method of determining the chemistry with x-ray spectroscopy proved to be very significant and reliable over the 5 year MER mission. The APXS on both MER rovers documented the overall geological context of the landing sites, identified stratigraphic sequences along the traverse and identified local anomalies, many of them were pointing towards interactions with water in the past. For the upcoming MSL rover mission the APXS method was further improved and now is able to take a full chemical analysis within about 3 hours, an improvement over the MER APXS by a factor of 3. Furthermore the temperature range for good resolution x-ray spectra was extended upwards to approximately-5C, whereas the MER APXS is only capable of functioning sufficiently below-40C. The MSL flight instrument underwent all qualification tests and a final science calibration before being delivered to NASA/JPL in November 2008. The Main electronics, the sensor head, as well as the basaltic calibration target are shown below.