Arguably, the most important of the first scientific instruments for Magellan will be IMACS, a combination wide-field imager and versatile low-to-medium dispersion spectrograph. The first Magellan telescope is being fitted with an f/11 Gregorian secondary mirror specifically designed to feed this instrument, by which it achieves excellent image quality and transmission. The on-axis optical performance, for small fields or single objects, will rival the best available at any telescope. However, it is the exceptionally wide field of 30 arc-minutes that distinguishes this instrument, making feasible two kinds of projects: (1) studies of rare objects, for example, photometric/spectroscopy searches for the lowest-metal-abundance halo giants or high-redshift AGN, and (2) gathering large samples, for example, measuring the colors, redshifts, and star formation rates of thousands of faint field galaxies at z > 1, or collecting many hundreds of radial velocities for globular cluster stars with a broad range in mass. With its wide field, multi-slit masks, and an 8K x 8K pixel CCD detector, IMACS will be able to image large sky areas and acquire spectra of hundreds of such objects simultaneously, at resolutions running from a fraction of an angstrom to several angstroms, with many thousand pixels devoted to each spectra.