This chapter reviews theoretical and experimental studies on the magnetic properties of carbon nanotubes, including both single-wall nanotubes and multiwalled carbon nanotubes. It discusses the main effects of an external magnetic field on the electronic properties of carbon nanotubes, with the main focus on how the Aharonov–Bohm phase alters the electronic band structure, density of states, magnetic susceptibility, persistent currents, and magnetotransport properties of carbon nanotubes. The chapter describes experimental studies on carbon nanotubes in magnetic fields. One of the unique properties of carbon nanotubes is that their metallicity can be controlled by an external magnetic field applied parallel to the tube axis. Given the specific electronic structure of a carbon nanotube, it is relatively straightforward to show that the application of an external magnetic field modifies the band structure in a truly unique manner. The application of a magnetic field perpendicular to the tube axis leads to completely different effects.