Ex situ transmission electron microscopy (TEM) performed on catalytically grown multiwall carbon nanotubes identified two types of catalyst−nanotube wall interfaces. The interfaces consisted of crystalline domains with different orientations: twist and twin boundaries in correspondence with quasi-spherical particles closer to the nanotube base and tilt boundaries in correspondence with high aspect ratio, tapered particles further from the base. TEM suggests that the domain boundaries maintain a rather steady position coupled to the catalytic particles, whereas the carbon atoms precipitate along the nanotube axis away from the particles. It is concluded that the relative movement of the carbon atoms with respect to the dislocations comprising the nanotube domain boundary located at the catalyst−wall interface is a significant elementary process in nanotube crystal growth driven by surface diffusion. The results appear consistent with the concurrence of base and tip growth for the catalytic synthesis of ca...