Most of the properties of a single-wall nanotube depend on its diameter and, to a less extend, on its chiral angle also called the helicity of the structure. These two parameters, diameter and helicity, completely define the pair of Hamada indices (n, m) that characterize the atomic structure of a nanotube. The most powerful techniques that may give access to both diameter and helicity are STM and electron diffraction. Both techniques are reviewed in this paper. Emphasis is also given to the characterization of ropes of single-wall nanotubes.