In recent years there has been much interest in a newly found member of the fullerene family, carbon nanotubes. As a result of the small dimensions of these structures much of the structural investigation has involved the use of HREM images of the tubes1−3. From these images it has been proposed that the idealised structure of a carbon nanotube comprises closed cylindrical coaxial shells of graphite in which each shell may have its own unique helical pitch due to the numerous ways in which a sheet of graphite may be rolled to form a cylinder. We have previously discussed HREM image simulations of the {100} fringes at the centre of nanotubes which assumed that the top and bottom faces of the tube were parallel4. In this paper attention is focused on image simulations for complete ideal tubes of circular cross-section in order to elucidate further the structure of the tubes. However, due to the complexity and large range of possible configurations and orientations of the tubes, we have not attempted to match the simulations to a particular tube. Instead we have chosen one representative structure and have varied the parameters of the tube in order to observe the effects on the image of the stacking of the shells relative to each other, tilt of the tube along the long axis and helical pitch of the shells. Features which occur in the simulations are compared qualitatively to experimental images.