Gentoo Astronomy Project Montage is a toolkit for assembling astronomical images into custom mosaics. It uses algorithms that preserve the calibration and positional (astrometric) fidelity of the input images to deliver mosaics that meet user-specified parameters of projection, coordinates, and spatial scale. It supports all projections and coordinate systems in use in astronomy. It contains independent modules for analyzing the geometry of images on the sky, and for creating and managing mosaics; these modules are powerful tools in their own right and have applicability outside mosaic production, in areas such as data validation. Caltech-IPAC/Montage