Multiple excitations of atomic electrons

The development of nonperturbative techniques to deal with multiple-electron perturbations in atomic spectra is reviewed. Through the joint use of quantum-defect theory and of a finite-volume channel interaction method, numerous spectral regularities have emerged. The analysis of hyperspherical coordinate maps for many-electron atoms extends many of our intuitive notions about separable systems to strongly correlated electron motion, which is inherently nonseparable.