IMaX (Imaging Magnetograph eXperiment) is one of the postfocal instruments for the SUNRISE experiment. The experiment consists of the launch from the Antarctic of a stratospheric globe, which will house a 1 m solar telescope with postfocal instrumentation. The United States (through NASA and two research institutes) is a partner in the SUNRISE project, with Germany (through the DLR and two research institutes one of which, the MPAe, is leading the project) and Spain (with the IMaX instrument). IMaX will provide data of an unprecedented quality on the sun's magnetic field: it will combine high temporal cadence with polarimetric accuracy, preserving the two-dimensional integrity of images. IMaX will be used to study the evolution and dynamics of solar magnetic fields at spatial and temporal resolutions never before achieved from earth. IMaX is being built by a consortium of four Spanish institutions with wide experience of participation in space projects - the IAC (Tenerife, coordinating), IAA (Granada), GACE (University of Valencia), and LINES/INTA (Madrid). The consortium is building IMaX in Spain with Spanish companies. IMaX uses liquid crystal based optical retarders designed by the IAC in collaboaration with the company TECDIS Display Ibérica (Valladolid). The instrument will be assembled at INTA premises in Madrid in 2007.