Ion irradiation with doses exceeding the amorphization threshold converts carbonaceous materials into amorphous carbon, which may possess high electrical conductivity and considerably different chemical activity. This effect is especially pronounced in diamond, which is an insulating and chemically inert material. Because of very sharp threshold of the ion-induced diamond-carbon conversion, the interface between the amorphized carbon and the intact substrate may be as narrow as a few nanometers. Thus using controlled scanning of the ion beams focused at nanoscale, one can reproducibly fabricate complex carbon nanostructures of predetermined 2D geometry. It has been shown that the focused ion beam-written carbon nanostructures possess novel electronic properties: non-linear behavior, electronic switching, anomalously high conductivity, Coulomb blockade at elevated temperatures. These structures can also form regular hydrophobic-hydrophilic nanopatterns or functionalized nanoarrays, which may be used for 2D self-assembly of macromolecules and polymers. The FIB-written carbon nanostructures are discussed as novel devices for carbon-based nanoelectronics and sensorics.