The atomic structure of the GaAs(001) surface has been disputed since molecular beam epitaxy (MBE) technique was developed in the earlier nineteen sixties. The invention of scanning tunneling microscopy (STM) with its real-space atom-resolution capability, has revolutionized the situation. This paper reviews the STM investigations of the principal reconstructions found on the GaAs(001) surface, As-rich 2x4 and 2x6, Ga-rich 4x2 and 4x6. These studies, together with advanced theoretical analyses, have finally resulted in establishment of a unified structural model for various reconstructions, with which we can explain most of the observations and long-standing controversies about the atomic structures and surface stoichiometries
