The propagation of electrons and atoms in external fields gives rise to interference effects, which have profound impact on the spectra and evolution of the underlying system. For example, the photoionisation rate of atoms and the conductivity through nanodevices are strongly modulated and controllable by electric and magnetic fields. I show analytic results for the energy-dependent Green's function and discuss their applications to atomic system, atom lasers from Bose-Einstein-condensates, and the Quantum Hall Effect.