Nucleation and growth of carbon nanotubes, formed during chemical vapor deposition, was observed using an environmental transmission electron microscope (ETEM). Carbon nanotubes formed readily when acetylene (C₂H₂) was leaked into the sample area of the ETEM containing Ni/SiO₂ catalyst at 470-700°C. Carbon nanotubes were observed to nucleate and grow in a sequential manner, i.e. one or two tubes nucleating and growing before new tubes were nucleated. Straight single wall tubes were observed to grow with no catalyst at the tip at 475°C at very low C₂H₂ gas pressures (< 10 mTorr). We observe that the growth rate and growth mechanism are directly controlled by the temperature and pressure. [DOI: 10.1380/ejssnt.2006.460]