Abstract: The search for two-dimensional (2D) allotropes of boron has attracted broad interest in the past decades, and many theoretical papers have predicted their structures and physical properties. Recently, 2D boron was successfully synthesized for the first time by our group. In this review we will discuss 2D boron, starting from its theoretical prediction, followed by a description of its atomic structures, method of synthesis, and characterization of physical properties such as electronic structure and chemical stabilization.