Halo nuclei, stepping stones across the drip-lines

The availability of intense secondary beams in conjunction with modern efficient detection setups allows for the production and detailed study of the most extreme nuclear systems, in terms of asymmetry of proton and neutron number, in the continuum. Nuclei close to the drip-lines, exhibiting exotic properties themselves, can be used as a basis in order to populate these even more exotic nuclear systems, e.g. in transfer and knockout reactions. The latter challenge nuclear structure theory by being open quantum systems far from the valley of beta stability as well as reaction aiming at a description of their production mechanisms. Experiments provide data on momentum distributions, while relative energy spectra, and spin alignment during the reaction can be extracted and lead to the observation of energy and angular correlations as well as to dependent quantities such as, e.g., the profile function denoting a momentum width as a function of relative energy. They are determined from reaction products and gamma radiation emerging from the reaction zone. The link to intrinsic properties of these unbound systems has to be explored by gathering precise knowledge of the properties of the seed nuclei and compare them to structures observed in the continuum. In this paper, examples of the above-mentioned methods are presented. The current knowledge about light systems such as 5,7 H, 7−10 He, 10−13 Li and the most neutron-rich oxygen systems is reviewed.