  The main purpose of the present manuscript is to review the structural
evolution along the isotonic and isotopic chains around the "traditional" magic
numbers 8; 20; 28; 50; 82 and 126. The exotic regions of the chart of nuclides
have been explored during the three last decades. Then the postulate of
permanent magic numbers was de nitely abandoned and the reason for these
structural mutations has been in turn searched for. General trends in the
evolution of shell closures are discussed using complementary experimental
information, such as the binding energies of the orbits bounding the shell
gaps, the trends of the rst collective states of the even-even semi-magic
nuclei, and the behavior of certain single-nucleon states. Each section is
devoted to a particular magic number. It describes the underlying physics of
the shell evolution which is not yet fully understood and indicates future
experimental and theoretical challenges. The nuclear mean eld embodies various
facets of the Nucleon- Nucleon interaction, among which the spin-orbit and
tensor terms play decisive roles in the shell evolutions. The present review
intends to provide experimental constraints to be used for the re nement of
theoretical models aiming at a good description of the existing atomic nuclei
and at more accurate predictions of hitherto unreachable systems.
