  A brief review of the manifestations of classical chaos observed in atomic
systems is presented. Particular attention is paid to the analysis of atomic
spectra by periodic orbit-type theories. For diamagnetic non-hydrogenic Rydberg
atoms, the dynamical explanation for observed spectral features has been
disputed. By building on our previous work on the photoabsorption spectrum, we
show how, by the addition of diffractive terms, the spectral fluctuations in
the energy level spectrum of general Rydberg atoms can be obtained with
remarkable precision from the Gutzwiller trace formula. This provides further
evidence that non-hydrogenic systems are most naturally described in terms of
diffraction rather than classical chaos.
