Eventhough the growth mechanism of carbon nanotubes is not very clear it is possible to suggest different mechanisms for the formation of CNTs. A mechanisms with three steps relates to first formation of nanotubes and fullerenes, C2, on the surface of the metal catalyst particle, then formation of a rodlike carbon of metastable carbide particle fallowed by a slow graphitisation of its wall. This mechanism is suggested based on in-situ TEM observations. Another mechanism postulates that metal catalyst particles can be either floating or supported on graphite or another substrate. If the catalyst particles are spherical or pear-shaped the deposition will take place on one half of the surface which is on the lower curvature side for the particles. The carbon diffuses along the concentration gradient and precipitates on the opposite half around and below the bisecting diameter. However, it does not precipitate from the apex of the hemisphere, which accounts for the hollow core that is characteristic of these filaments. For supported metals, filaments can be formed either by extrusion or base growth in which the nanotube grows upwards from the metal particles that remain attached to the substrate, or the particles detach and move at the head of the growing nanotube by the tip-growth. Depending on the size of the catalyst particles, SWNT or MWNT are grown. In arc discharge, if no catalyst is present in the graphite, MWNT will be grown on the C2-particles that are formed in the plasma.