[en] Triple differential cross sections for the ionization of atomic hydrogen in the intermediate energy regime are discussed in terms of theoretical approximations based on the Born series (B1, B2, EBS, and DWBA) and on the multple scattering expansion (DWIA). First order plane wave theories are incapable of describing the data, and the inclusion of higher order effects is essential. The impulse approximation works very well in the high momentum transfer region, whereas the Born series approach works best in the region of very small momentum transfer. Some fundamental difficulties remain in the intermediate region