Carbon nanotubes are quasi one-dimensional nano-structures with unique electrical properties that make them prime candidates for molecular electronics, which is certainly a most promising direction in nano-technology. Early theoretical works predicted that the electronic properties of "ideal" carbon nanotubes depend on their diameter and chirality. However, carbon nanotubes are probably not as perfect as they were once thought to be. Defects like pentagons, heptagons, vacancies, are found to modify drastically the electronic properties of these nano-systems. The introduction of defects in the carbon network is thus an interesting way to tailor its intrinsic properties, in order to create carbon nanotube based nano-devices, leading to new potential applications in nano-electronics.