GaAs(001) is one the most commonly used substrates in fabrication of wireless and opto-electronic devices based on III-V compound semiconductors by using molecular beam epitaxy (MBE), metallorganic chemical vapor deposition (MOCVD) and related techniques. The surface structure of GaAs(001) has been disputed since the beginning of the development of the techniques as to which of these materials are artificially prepared. The invention of scanning tunneling microscopy (STM) has revolutionized the situation. This paper reviews the STM studies of principal reconstructions, from As-rich c(4×4), 2×4, 2×6 to Ga-rich 4×2 and 4×6, found on the GaAs(001) surface. These studies, together with advanced theoretical efforts, have eventually resulted in establishment of a unified model for various reconstructions, with which we could explain most of the observation and long-standing controversies in atomic structures and surface stoichiometries.