Carbon nanotubes have the capability to contain or transport other atoms, molecules, or crystalline materials inside. The constrained lumen of a nanotube could be a novel reaction chamber, catalyzing the synthesis of new compounds by forcing reactants and products to adopt specific conformations. This chapter discusses advances in the creation of nanotube heterostructures. It explores the synthesis of the one-dimensional materials and how their structures are manifest in their unique properties. The chapter describes advances in the synthesis and characterization of related double-wall carbon nanotube materials. The deceptively simple one-dimensional structure of a carbon nanotube gives rise to a host of fascinating mechanical, electrical, field emission, and optical properties. Initial efforts to create hierarchial nanotube materials focused on filling multiwall carbon nanotubes with metal complexes and related species. X-ray diffraction is typically applied to single-wall carbon nanotubes to determine the crystalline ordering of nanotube bundles.