  Charged plasma particles form an anisotropic counter-streaming distribution
at the front of a shock. In the near- and ultra-relativistic regimes, Weibel
(or two-stream) instability produces near-equipartition, chaotic, small-scale
magnetic fields. The fields introduce effective collisions and "thermalize" the
plasma particles via pitch-angle scattering. The properties of jitter radiation
emitted by accelerated electrons from these small-scale fields are markedly
different from synchrotron spectra. Here we theoretical summary and the results
of recent numerical 3D PIC plasma kinetic simulations. The relation of the
obtained results to the theory of gamma-ray bursts is outlined.
