Electrons in Superfluid Helium-4

We review the current status of theory and experiment concerning electrons in superfluid helium-4 with an emphasis on some of the unsolved problems. Although calculations of the bubble structure have been confirmed in detail by measurements of the optical absorption spectrum, the energy of interaction of a bubble with a vortex is not understood. We discuss the theory behind a proposed experiment that may shed light on the physical nature of the exotic ions.