Halo nuclei are characterized by an approximate decoupling - spatially and dynamically - of one or two nucleons from the remaining nucleus. This lecture identifies the key features of halo systems and reviews scaling relations that enable halos in different systems to be compared. Regions where halos have been seen or are likely to be found are outlined. A general overview is given of how halos are studied experimentally followed by a more detailed discussion on nuclear reaction experiments at energies above 20 MeV/u and on halo effects in beta decay.