Montage is a portable, scaleable toolkit that runs on end-users desktops, clusters, and computing grids. It generates astronomical image mosaics that preserve the calibration and photometry in the input FITS files. The code is available for download at the project website at <http://montage.ipac. caltech.edu>, and includes independent modules for image discovery (including support for VO image access protocols), image reprojection, rectification of the sky background to a common level and image co-addition.