  Relativistic shocks are usually thought to occur in violent astrophysical
explosions. These collisionless shocks are mediated by a plasma kinetic
streaming instability, often loosely referred to as the Weibel instability,
which generates strong magnetic fields "from scratch" very efficiently. In this
review paper we discuss the shock micro-physics and present a recent model of
"pre-conditioning" of an initially unmagnetized upstream region via the
cosmic-ray-driven Weibel-type instability.
