  Particle acceleration in collisionless astrophysical shocks, i.e., diffusive
shock acceleration (DSA), is the most likely mechanism for producing cosmic
rays, at least below 10^{15} eV. Despite the success of this theory, several
key elements, including the injection of thermal particles, remains poorly
understood. We investigate injection in strongly nonlinear shocks by comparing
a semi-analytic model of DSA with a Monte Carlo model. These two models treat
injection quite differently and we show, for a particular set of parameters,
how these differences influence the overall acceleration efficiency and the
shape of the broad-band distribution function.
