  Solving the continuum radiative transfer equation in high opacity media
requires sophisticated numerical tools. In order to test the reliability of
such tools, we present a benchmark of radiative transfer codes in a 2D disc
configuration. We test the accuracy of seven independently developed radiative
transfer codes by comparing the temperature structures, spectral energy
distributions, scattered light images, and linear polarisation maps that each
model predicts for a variety of disc opacities and viewing angles. The test
cases have been chosen to be numerically challenging, with midplane optical
depths up 10^6, a sharp density transition at the inner edge and complex
scattering matrices. We also review recent progress in the implementation of
the Monte Carlo method that allow an efficient solution to these kinds of
problems and discuss the advantages and limitations of Monte Carlo codes
compared to those of discrete ordinate codes. For each of the test cases, the
predicted results from the radiative transfer codes are within good agreement.
The results indicate that these codes can be confidently used to interpret
present and future observations of protoplanetary discs.
