This review describes advances in understanding the structural, electr
onic, and chemical properties of clean low-index semiconductor surface
s during the first decade following the advent of the scanning tunneli
ng microscope (STM). The principles of STM are discussed together with
the instrumentation required to perform STM measurements on semicondu
ctor surfaces in ultrahigh vacuum. A comprehensive review of the struc
tures of the clean, low-index surfaces of elemental and compound semic
onductors is presented. These structures are discussed using the gener
al physical principles that determine them.