High-resolution studies of the Sun’s magnetic fields are needed for a better understanding of solar magnetic fields and the fundamental processes responsible for solar variability. The generation of magnetic fields through dynamo processes, the amplification of fields through the interaction with plasma flows, and the destruction of fields are still poorly understood. There is still incomplete insight as to what physical mechanisms are responsible for heating the corona, what causes variations in the radiative output of the Sun, and what mechanisms trigger flares and coronal mass ejections. Progress in answering these critical questions requires study of the interaction of the magnetic field and convection with a resolution sufficient to observe physical scales fundamental to these processes. The 4m aperture ATST will be a unique scientific tool, with excellent angular resolution, a large wavelength range, and low scattered light. With its integrated adaptive optics, the ATST will achieve a spatial resolution nearly 10 times better than any existing solar telescope. The ATST will provide:Unprecedented angular resolution of 0.03 arcsec in the visible and 0.08 arcsec at 1.6 microns to enable us to clearly resolve and study the fundamental astrophysical processes on their intrinsic scales and to verify model predictions. A high photon flux for accurate and precise measurements of physical parameters, such as magnetic field strength and direction, temperature and velocity, on the short time scales involved. Access to a broad set of diagnostics, from visible to thermal infrared wavelengths. Low scattered light observations and coronagraphic capabilities in the infrared, allowing measurements of coronal magnetic fields. The ATST has been highly ranked by the latest Decadal Survey of Astronomy and Astrophysics and the NAS/NRC study of ground-based solar astronomy. A large part of the solar community will participate in the design and development of the ATST. A strawman telescope design, design challenges and instrument concepts will be discussed. Examples of recent high resolution observations with adaptive optics, that demonstrate the potential of this new technology will be shown.