The 4m aperture Advance Technology Solar Telescope (ATST) will be the most powerful solar telescope in the world and a unique scientific tool to study the Sun and other astronomical objects, such as planets. The ATST will replace major existing national solar facilities at the end of this decade. The ATST project has entered the design and development phase. We present an overview of the ATST science drivers and discuss preliminary design concepts and technical challenges. The ATST science goals lead to the following general requirements for the ATST facility: - Diffraction limited angular resolution in the visible and infrared to study fundamental astrophysical processes with unprecedented resolution enabling verification of model predictions. - A high photon flux for accurate measurements of physical parameters throughout the solar atmosphere, such as magnetic strength and direction, temperature and velocity. - Access to a new diagnostics at relatively unexplored infrared wavelength. - Low scattered light to enable coronal observations. - Low instrumental polarization for accurate measurements of magnetic fields. Development of a 4m solar telescope presents many technical challenges. The large aperture drives the ATST to an open-air design and makes thermal control of optics and telescope structure a paramount consideration. To achieve diffraction-limited observations at visible and infrared wavelength ATST will have a high order solar adaptive optics system. 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. Preliminary telescope and instrument concepts will be discussed.