Nuclear collective motion: strength and limitations of group theroretical methods

We discuss both the strength of the Interacting Boson Model (IBM) as a unifying structure, emphasizing the robust elements as well as the limitations in an early U(6) group structure of interacting s and d bosons. We briefly address the rich structure of the IBM-2 approach, when treating proton and neutron degrees of freedom explicitly, which gives rise to non-symmetric spatial excitation modes of which a number have been detected experimentally. Finally, the very clear observation of particle-hole excitations near to closed-shell regions, leads to an extension of the IBM to include both particle and hole bosons. The basic concepts are presented as well as the compelling experimental evidence pointing out the existence of larger group-structures than the U(6) group structure.