Carbon forms a variety of allotropes (structures) due to the diverse hybridization of s- and p-electron orbitals, time-honored graphite (two dimensional structure) and diamond (three dimensional network), new forms such as C60 fullerene (cage structure), nanotube (quasi-one dimensional structure), graphene (truly two dimensional network), and carbyne (truly one dimensional chain). The family of new allotropes (fullerene, nanotube, graphene, carbyne) are called "nanostructured carbon" or "nanocarbon". They exhibit extreme properties, e.g., ultrahigh mechanical strength, ultrahigh charge carrier mobility, high thermal conductivity and so on, thus attracting considerable attentions for electronic and mechanical applications as well as in basic materials science for exploring new physics and chemistry. Carbon nanotube (CNT) and graphene possess excellent properties as electron field-emitters owing to their excellent properties. Electron sources are important in a wide range of areas from basic physics and scientific instruments to medical and industrial applications. CNT and graphene are unique as field emitters and offer several benefits compared to traditional cathodes (e.g., thermionic and photocathodes). Field emission produces very intense electron currents from a small surface area with a narrow energy spread, providing a highly coherent electron beam. The combination of the high current, the small source size and the narrow energy distribution not only provides us with the brightest electron sources but also explores a new field of electron-beam related researches. This book describes recent progress of researches on nanocarbon field electron emitters ranging from fundamental properties to promising applications, e.g., X-ray sources, vacuum electronic devices, space thrusters and so on.