Few-body effects in nuclear halos

Nuclear halos are spatially extended quantum states with large cluster comp onents. Characteristic observable properties of two- and three-body halo st ructures are discussed in general terms suited for generalization of the co ncept. Specific properties of known nuclear halos are used as examples. The bound-state structure is basically understood in contrast to the three-bod y continuum structure. The high-energy reaction mechanism is roughly unders tood but not accurately worked out and tested in details. Open questions an d challenges are indicated. (C) 2001 Elsevier Science B.V. All rights reser ved.