Halo states extend over an unusually large space where many of their properties hinge on the tail of the wave functions. Their levels lie mainly just below the thresholds for neutron-emission channels. This short review covers the main distinguishing features of halos and how they are revealed in experiments. Special emphasis is placed on the large variety of experiments apt to study halo structures. Some aspects of current research are treated in greater detail.

DOI:https://doi.org/10.1103/RevModPhys.66.1105