Physical conditions in the Orion Nebula and an assessment of its helium abundance

New long-slit spectrophotometric observations were obtained to redetermine the helium abundance of the Orion Nebula. The ionic ratio He(+)/H(+) is found to remain nearly constant at 0.088 + or -0.006. In the bright innermost region, the errors are largely systematic due to uncertainties in the reddening curve for Orion grains and deviations from case B emissivity. In the outer regions, the errors are dominated by statistical errors. Photoionization models are computed to determine the correction for the unobserved presence of neutral helium in regions where hydrogen is ionized.