This review reflects the status (in early 1973) of the problem of producing photoemitters and secondary and field emitters based on a new principle, that of obtaining semiconductor structures with negative electron affinity (NEA). The energy level scheme of the surface region of a semiconductor with NEA is examined, as well as the conditions for its realization. The main theoretical concepts of the mechanism of the emission from NEA emitters are developed, and the difference between their characteriistics and the corresponding characteristics of ordinary emitters are indicated. The technology is discussed of the production of NEA photocathodes for the visbile and infrared region of the spectrum, based on III-V semiconductor compounds (principally GaAs) and their solid solutions. Aspects of the production of semitransparent NEA photocathodes and secondary emitters operating in transmission are noted. The principal parameters of the experimental and commercial NEA photocathodes and secondary electron emitters are given. Data are reported on the use of surfaces with NEA in field-emission film cathodes, the operating principle of injection and optoelectronic field emission cathodes is described, and their present-day parameters are given.