2001PASP..113...29O

The nature of the structure of the Orion Nebula has become clearer but more complex. Quantitative application of simple photoionization theory has allowed construction of a three-dimensional model of the main ionization front, while high-resolution study of the flow of [O I], [O II], [O III], and [S III] has allowed determination of how this material moves. Material in a ``foreground lid'' of H I is seen in absorption lines in the 21 cm continuum and in optical spectra of Na I and Ca II. There remain many unsolved and possibly basic questions, among them the source of nonthermal broadening of all lines, which carries as much energy as the thermal content of the gas. We have also found that varying amounts of emission-line light scattered by the dust particles immediately behind the main ionization front introduces a nonphotometric scatter of up to 25% in spectral intensities.