A negative electron affinity (NEA) state is formed on the surfaces of Si and some III-V compound semiconductors, on which are adsorbed a Cs and/or Cs-0 monolayer. Since NEA surfaces exhibit a high rate of electron emission, many studies, on both the fundamental and practical levels, have been carried out. This paper reviews the the results of surface studies related to NEA. First, the physical principles of NEA operation and the preparation of clean surfaces for NEA ane explained. Included are the main results of studies on NEA surfaces of Si and GaAs, obtained by LEED, AES and photoemission spectroscopy. Finally, the states of Cs and Cs-O adsorbed on GaP surfaces as determined by the thermal desorption method are discussed and the optimum Cs vapor pressures for preparation of NEA GaP surfaces are presented.