  We present a comparison between independent computer codes, modeling the
physics and chemistry of interstellar photon dominated regions (PDRs). Our goal
was to understand the mutual differences in the PDR codes and their effects on
the physical and chemical structure of the model clouds, and to converge the
output of different codes to a common solution. A number of benchmark models
have been created, covering low and high gas densities and far ultraviolet
intensities. The benchmark models were computed in two ways: one set assuming
constant temperatures, thus testing the consistency of the chemical network and
photo-processes, and a second set determining the temperature selfconsistently.
We investigated the impact of PDR geometry and agreed on the comparison of
results from spherical and plane-parallel PDR models. We identified a number of
key processes governing the chemical network which have been treated
differently in the various codes, and defined a proper common treatment. We
established a comprehensive set of reference models for ongoing and future PDR
model bench-marking and were able to increase the agreement in model
predictions for all benchmark models significantly.
