A systematic study of closed classical orbits of the hydrogen atom in crossed
electric and magnetic fields is presented. We develop a local bifurcation
theory for closed orbits which is analogous to the well-known bifurcation
theory for periodic orbits and allows identifying the generic closed-orbit
bifurcations of codimension one. Several bifurcation scenarios are described in
detail. They are shown to have as their constituents the generic
codimension-one bifurcations, which combine into a rich variety of complicated
scenarios. We propose heuristic criteria for a classification of closed orbits
that can serve to systematize the complex set of orbits