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Magnetic fields control the inconstant Sun. The key to understanding solar variability and its direct impact on the Earth rests with understanding all aspects of these magnetic fields. The Advanced Technology Solar Telescope (ATST) has been design specifically for magnetic remote sensing. Its collecting area, spatial resolution, scattered light, polarization properties, and wavelength performance all insure ATST will be able to observe magnetic fields at all heights in the solar atmosphere from photosphere to corona. After several years of design efforts, ATST has been approved by the U.S. National Science Foundation to begin construction with a not to exceed cost cap of approximately $298M. Work packages for major telescope components will be released for bid over the next several months. An application for a building permit has been submitted. Science goalsSolar mass ejections, solar flares, solar wind, and variations in the solar irradiance are all causally related to the ever changing solar magnetic field. In a way, magnetic fields are the "dark energy" problem of solar physics. Only in the solar photosphere do we have direct measurements with some degree of accuracy. But even in the photosphere, the magnetic fields change so rapidly that current telescopes can not resolve many aspects of the field before it has evolved, and the bulk of the field may exist on scales that have not been resolved. In higher atmospheric layers, our current ability to accurately measure the magnetic field is rudimentary at best.The photosphere is a crucial region where energy is transformed readily from convective motion into thermal and magnetic energy, and electromagnetic radiation. The energy stored in magnetic fields is eventually dissipated at higher layers of the solar atmosphere, sometimes in the form of violent flares and coronal mass ejections (CMEs) that ultimately affect Earth and drive space weather. The photosphere, the chromosphere, transition region, and the corona are connected through the magnetic field and therefore have to be treated as one system, rather than as individual layers. The ATST, with high performance adaptive optics and state-of-the-art instrumentation, is a crucial tool to understand this complex, interconnected physical system. Some of the scientific problems the ATST will address include the origin and generation of magnetic fields, magnetic activity and instability, magneto-convection, chromospheric and coronal structure and heating, and sources of solar irradiance variability. A complete description of ATST science and its synergism with space missions is available at

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The 4m Advance Technology Solar Telescope (ATST) will be the most powerful solar telescope in the world, providing a unique scientific tool to study the Sun and possibly other astronomical objects, such as solar system planets. We briefly summarize the science drivers and observational requirements of ATST. The main focus of this paper is on the many technical challenges involved in designing a large aperture solar telescope. The ATST project has entered the design and development phase. Development of a 4-m solar telescope presents many technical challenges. Most existing highresolution solar telescopes are designed as vacuum telescopes to avoid internal seeing caused by the solar heat load. The large aperture drives the ATST to an open-air design, similar to night-time telescope designs, and makes thermal control of optics and telescope structure a paramount consideration. A heat stop must reject most of the energy (13 kW) at prime focus without introducing internal seeing. To achieve diffraction-limited observations at visible and infrared wavelengths, ATST will have a high order (order 1000 DoF) adaptive optics system using solar granulation as the wavefront sensing target. Coronal observations require occulting in prime focus, a Lyot stop and contamination control of the primary. An initial set of instruments will be designed as integral part of the telescope. First telescope design and instrument concepts will be presented.