The planned Advanced Technology Solar Telescope (ATST) will be a 4-m aperture general-purpose solar telescope with integrated adaptive optics and versatile post focus instrumentation. The ATST will achieve an angular resolution of 0.03 arcsec (20 km on the solar surface) in the visible, which is almost an order of magnitude better than what is achieved with current solar telescopes. This will make it possible to resolve the fundamental astrophysical hydrodynamic and magnetohydrodynamic processes and structures in the solar atmosphere such as the building blocks of solar magnetic fields that are believed to be responsible for solar irradiance variations and the heating of the outer solar atmosphere. The ATST will cover the wavelength range from 0.35 to 35 ?m and minimize scattered light. The initial set of post focus instruments will exploit the unique capabilities of the ATST to study magnetic fields at the highest spatial resolution in the visible and near-infrared parts of the spectrum. The ATST was highly recommended by the recent Decadal Study. A proposal for a four-year Design and Development phase has just been submitted to the NSF. Construction is expected to start in FY2005. The National Solar Observatory is operated by the Associated Universities for Research in Astronomy and is funded by the National Science Foundation under a cooperative agreement.