The Giant Magellan Telescope (GMT) is a joint project of a consortium of universities and research institutions to build and operate a 21.5-m equivalent aperture astronomical telescope for use at visible and IR wavelengths. This paper briefly summarizes the science goals for the project and provides an overview of the preliminary telescope and enclosure concepts and site test program. The telescope is a Gregorian design with a fast, f/0.7, primary mirror that allows a compact and stiff mount structure. The 25.3-meter diameter primary mirror consists of six off-axis 8.4-meter circular mirrors arranged in a hexagon around a center 8.4-meter mirror. The Gregorian secondary mirror is adaptive allowing two-mirror, wide-field adaptive optics. Several corrector designs have been studied for wide-field applications and one such design is shown. Instruments being considered for GMT provide a wide range of scientific capabilities. Instruments mount below the primary mirror on an instrument platform. Instrument mounting and servicing provisions are summarized.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.