  We have developed a three dimensional Monte Carlo photoionization code
tailored for the study of Galactic H II regions and the percolation of ionizing
photons in diffuse ionized gas. We describe the code, our calculation of
photoionization, heating & cooling, and the approximations we have employed for
the low density H II regions we wish to study. Our code gives results in
agreement with the Lexington H II region benchmarks. We show an example of a 2D
shadowed region and point out the very significant effect that diffuse
radiation produced by recombinations of helium has on the temperature within
the shadow.
