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. For example, the generation of magnetic fields through dynamo processes is 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. Progress in answering these critical questions requires study of the interaction of the magnetic field and convection with a resolution sufficient to observe scales fundamental to these processes. The 4m aperture ATST will be a unique scientific tool, which will provide unprecedented angular resolution, high photon flux, access to a broad set of diagnostics, from visible to thermal infrared wavelengths, and low scattered light observations and coronagraphic capabilities in the infrared. Development of a 4-m solar telescope presents several technical challenges. The large heat flux makes thermal control of optics and telescope structure a paramount consideration. To achieve diffraction-limited performance, a powerful solar adaptive optics system is required. Low scattered light is essential for observing the corona but also to accurately measure the physical properties of small structures in, for example, sunspots. Contamination control of the primary and secondary mirrors must therefore be addressed. An initial set of instruments will be designed as integral part of the telescope during the upcoming design and development phase. A strawman telescope design and instrument concepts will be discussed.