The constraints imposed by X-ray, ultraviolet, and optical observations on the absorbing material in the Seyfert 1.5 galaxy NGC 4151 are discussed. These observations are compared to the predictions of photoionization models of both broad line region clouds, which Holt et al. identify as the source of the extinction, and to models of the narrow line region gas. The weakness of both C III] 1909 and the broad component of Hα relative to C IV 1549, together with the absence of an absorption Balmer jump, indicate that the ionization parameter for the broad line region is a factor ~ 30 times larger in NGC 4151 than is found in typical QSO clouds. The energy budget of the system, as deduced from model calculation and emission-line fluxes, demands essentially full coverage of the continuum source by both broad and narrow line region clouds. The continuum incident on the outer narrow line region clouds is dominated by hard (hν \u3e 5 keV) X-rays because of the effective low frequency absorption arising in the inner broad line region gas. The calculations suggest that narrow line region clouds have a composition similar to extragalactic giant H II regions, and the hydrogen line spectrum shows that dust is mixed with narrow line region gas. We argue that broad line region clouds are likely to be in a radiatively driven wind and show that these clouds are opaque to radio emission. Some other consequences of these calculations are also discussed
