Highly excited, or Rydberg, atoms interact strongly with external perturbations, providing Prof. Gallagher´s group with opportunities to quantitatively explore otherwise inaccessible phenomena. They are studying the interaction of Rydberg atoms with microwave fields which have frequencies comparable to the Kepler orbital frequency of the electron, a regime in which many phenomena exhibit both classical and quantum behavior. They are examining the properties of relatively high density samples of cold Rydberg atoms using a magneto optical trap. These samples can behave as disordered solids or, under only slightly different conditions, evolve into an ultracold plasma. Understanding these fascinating systems provides many challenges. Finally, Prof. Gallagher´s group is studying autoionization and its inverse, recombination, in atoms with two optically active electrons. The effects of external perturbations, such as fields, on the energy transfer between electrons is being examined.