Carbon nanotubes are cylindrical carbon molecules with properties that make them potentially useful in extremely small scale electronic and mechanical applications. They exhibit unusual strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized.

A nanotube has a structure similar to a fullerene, but where a fullerene molecule is spherical, a nanotube is cylindrical, with one end typically being capped with half a fullerene molecule. Their name derives from their size; nanotubes are on the order of only a few nanometres wide (on the order of one ten-thousandth the width of a human hair), and their length can be millions of times greater than their width. There are two main types of nanotubes: single-walled nanotubes (SWNT) and multi-walled nanotubes (MWNT).

Nanotubes are composed entirely of sp² bonds, similar to graphite. Stronger than the sp3 bonds found in diamond, this bonding structure provides them with their unique strength. Nanotubes naturally align themselves into “ropes” held together by Van der Waals force. Under high pressure, nanotubes can merge together, trading some sp2 bonds for sp3 bonds, giving great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking.

While it has long been known that carbon fibers can be produced with a carbon arc, and patents were issued for the process, it was not until 1991 that Sumio Iijima, a researcher with the NEC Laboratory in Tsukuba, Japan, observed that these fibers were hollow. This feature of nanotubes is of great interest to physicists because it permits experiments in one-dimensional quantum physics.