The frontiers of our knowledge of the nuclear world advance continuously following progress in experimental techniques. The quest to reach very unstable nuclei at both the neutron-rich and the neutron-deficient limits of the chart of nuclei is motivated by expectations that the structure of an atomic nucleus changes when the imbalance between the number of neutrons and the number of protons increases. New features and new phenomena are expected. These generally pose a challenge to theoretical models that were developed to describe, and thoroughly tested with, nuclei in the vicinity of stable ones.