  We model the spectral energy distribution of the type 1 Seyfert galaxy NGC
5548, fitting data from simultaneous optical, UV, and X-ray monitoring
observations. We assume a geometry consisting of a hot central Comptonizing
region surrounded by a thin accretion disk. The properties of the disk and the
hot central region are determined by the feedback occurring between the hot
Comptonizing region and thermal reprocessing in the disk that, along with
viscous dissipation, provides the seed photons for the Comptonization process.
The constraints imposed upon this model by the multiwavelength data allow us to
derive limits on the central black hole mass, $M \la 2 \times 10^7$ \Msun, the
accretion rate, $\Mdot \la 2.5\times 10^5$ M$_\odot^2$yr$^{-1}/M$, and the
radius of the transition region between the thin outer disk and the
geometrically thick, hot inner region, $\sim 2$--5$\times 10^{14}$ cm.
