  First order Fermi acceleration at astrophysical shocks is often invoked as a
mechanism for the generation of non-thermal particles. This mechanism is
especially simple in the approximation that the accelerated particles behave
like test particles, not affecting the shocked fluid. Many complications enter
the calculations when the accelerated particles have a backreaction on the
fluid, in which case we may enter the non linear regime of shock acceleration.
In this paper we summarize the main features of a semi-analytical approach to
the study of the non linearity in shock acceleration, and compare some of the
results with previous attempts and with the output of numerical simulations.
