The confinement of molecules or materials in carbon nanotubes (CNTs) may generate unique one-dimensional supramolecular nanostructures that may bring forth novel electronic, magnetic, and chemical properties. The nature of the confinement effects can be either physical or chemical. The physical nature is related to space restriction in the CNT channels. The chemical factors are related to the chemical nature of the pores, which influences the interaction of the confined phase with the pore wall. Apparently all of these effects can influence chemical transformations, catalytic or not, with the extent of the influence depending on the selected catalysts (if any), the diameter of CNTs, and the specific reactions being carried out. Although wide efforts have been made to assess the relative importance of each factor in order to gain fundamental insights, they are still, in most cases, rather difficult to clearly identify.