  A critical review is presented of the current status of our understanding of
the the spectral output of thermally-radiating accretion disks around black
holes, with special attention to those around supermassive black holes in
active galactic nuclei. Uncertainties in disk physics (e.g. the nature of the
inter-ring stresses, the distribution of dissipation with altitude) contribute
to uncertainties in the output. So, too, do inadequacies in calculations (e.g.
incomplete treatment of non-LTE effects, neglect of transitions in elements
heavier than He, difficulty of coupling Comptonization to genuine transfer).
Existing models do not yet adequately explain the absence of Lyman edge
features, or the observed flux in the EUV.
