  The atomic nucleus is a quantum many-body system whose constituent nucleons
(protons and neutrons) are subject to complex nucleon-nucleon interactions that
include spin- and isospin-dependent components. For stable nuclei, already
several decades ago, emerging seemingly regular patterns in some observables
could be described successfully within a shell-model picture that results in
particularly stable nuclei at certain magic fillings of the shells with protons
and/or neutrons: N,Z = 8, 20, 28, 50, 82, 126. However, in short-lived,
so-called exotic nuclei or rare isotopes, characterized by a large N/Z
asymmetry and located far away from the valley of beta stability on the nuclear
chart, these magic numbers, viewed through observables, were shown to change.
These changes in the regime of exotic nuclei offer an unprecedented view at the
roles of the various components of the nuclear force when theoretical
descriptions are confronted with experimental data on exotic nuclei where
certain effects are enhanced. This article reviews the driving forces behind
shell evolution from a theoretical point of view and connects this to
experimental signatures.
