  We discuss some aspects of the propagation of high-energy cosmic rays (CRs)
in turbulent magnetic fields, and propose a formula for the diffusion
coefficient based on accurate simulations in a wide energy range. We discuss
the transition between ballistic and diffusive regimes and the magnetic horizon
of CR sources, showing how magnetic fields of a few nG could modify the shape
of the GZK feature. Such fields would roughly be in equipartition with the
extragalactic CRs, and could build through the resonant growth of waves within
the lifetime of the universe.
