  After a successful development of theoretical and numerical works on Fermi
acceleration at relativistic shocks, some difficulties recently raised with the
scattering issue, a crucial aspect of the process. Most pioneering works were
developed assuming the scattering off magnetic fluctuations as given. Even in
that case, when a mean field is considered, its orientation is mostly
perpendicular to the shock normal in the front frame, and this tends to quench
the scattering process. Solving this difficulty leads to address the issue of
the generation of very intense magnetic fluctuations at short wave lengths. The
relativistic motion of the shock front let the cosmic rays to visit upstream
during a very short time only, making this generation of magnetic fluctuations
very challenging. Anyway there is some hope to solve the problem. Thanks to a
recent work by Spitkovsky (2008) \cite{AS}, we know that the process works
without any mean field and now we have to investigate up to which intensity the
mean field can be amplified for allowing Fermi process with appropriate fast
instabilities. In this presentation, the collisionless shock structure in
relativistic regime is sketched, the scattering issue is presented, and the
instabilities that can provide the expected magnetic field amplification are
presented as well. Although there exists observational evidence that particles
are accelerated in relativistic flows and are distributed according to a power
law suggesting a Fermi process, the drastic conditions for Fermi process to
work are not always clearly fulfilled.
