Multi-particle multi-hole excitations and new symmetries near closed shells

We discuss the experimental evidence for many-particle many-hole excitations (mp-nh) at and near to closed shells. Besides a shell-model approach and in order to understand the salient features of the general behaviour of the 0+ intruder band heads, we mainly concentrate on a group theoretical classification. Using the boson model, various possibilities to classify intruder mp-nh excitations can be shown to exist. Ample evidence for the realization of these new symmetries is given in (i) the Z = 50 mass region and, in (ii) the Z = 82 Pb mass region. Finally, a possible extension to encompass also mp-nh excitations in light nuclei is presented. (orig.)