  Theoretical concepts ofn cosmic ray particle acceleration at relativistic
plasma flows -- shocks and shear layers -- are reviewed. We begin with a
discussion of mildly relativistic shock waves. The role of oblique field
configurations and field perturbations in forming the particle energy spectrum
and changing the acceleration time scale is considered. Then, we report on two
interesting attempts to consider particle acceleration at ultra-relativistic
shocks. Finally, in contrast to the compressive shock discontinuities, we
discuss the acceleration processes acting in the boundary layer at the
tangential velocity transition. The second-order Fermi acceleration as well as
the cosmic ray `viscous' acceleration provide the mechanisms generating
energetic particles there.
