Carbon nanotubes, a novel form of carbon discovered in 1991, have been rapidly recognized as one of the most promising electron field emitters ever since the first emission experiments reported in 1995. Their potential as emitters in various devices has been amply demonstrated during the last five years, and recent developments of production techniques are likely to trigger future applications. This report reviews the state of the art of the current research on the electron field emission properties of carbon nanotubes and surveys their ability to provide single or multiple electron sources.