  In this proceedings we discuss recent findings regarding the large order
behavior of the Chapman-Enskog expansion in relativistic kinetic theory. It is
shown that this series in powers of the Knudsen number has zero radius of
convergence in the case of a Bjorken expanding fluid described by the Boltzmann
equation in the relaxation time approximation. This divergence stems from the
presence of non-hydrodynamic modes, which give non-perturbative contributions
to the Knudsen series.
