When multi-GeV electrons and positrons penetrate single crystals near axial/planar directions dramatic effects appear. The large gamma factors (105−106) enhance the electric fields along axes/planes to overcritical values within the electron restframe. Overcritical in this context means that perturbation theory (used hitherto) is no longer applicable, but a full description of QED in strong fields has to be used. Bremsstrahlung becomes a highly non-perturbative process resulting in a much enhanced cross section. Originally a peak in the photon spectra from 150 GeV electrons penetrating Ge-crystals was detected. In the literature different suggestions for processes leading to this peak is found, i.e. “new” physics, radiation cooling, multiphotons etc. Along axial/planar directions these effects were investigated in detail by our CERN-collaboration (NA-43) and surprising effects were found. 150 GeV axially channeled electrons radiate 80% of the total energyEoin 1 mm Ge – 20 times more than in amorphous material. The peak in the photon spectra around (0.8–0.9) ×Eois a multiphoton effect and found both in Si and Ge, but not for thin Si-crystal and not for incident angles larger than ∼ 0.5 × the channeling angle ψ1. The influence of strong fields on shower formation was also measured along axial directions. Radiation lengths are (10–50) times shorter than Bethe-Heitler values. The angular dependence is in agreement with the critical angle θ0=Uo/mc2∼ 0.5 mrad calculated by Baier et al. for strong field effects.