Strong and sustained electron emission at low electric fields was observed in undoped, nanostructured diamond. Electron emission of 10 milliamperes per square centimeter was observed at applied fields of 3 to 5 volts per micrometer, These are the Lowest fields ever reported for any field-emitting material at technologically useful current densities. The emitter consists of a layer of nanometer-size diamond particulates, which is heat-treated in a hydrogen plasma. These emission characteristics are attributed to the particles＇ high defect density and the low electron affinity of the diamond surface. Such emitters are technologically useful, because they can be easily and economically fabricated on large substrates.