Abstract Most Seyfert 1 nuclei and quasars show strong excess continuum flux in the blue and ultraviolet, relative to an extrapolation of their spectra at longer wavelengths. The arguments for identifying this “Big Blue Bump” as thermal emission from an optically thick accretion flow are outlined. Further (less secure) arguments are presented that the flow is flattened, possibly in a disc. The close agreement between simple accretion disc models and the observations is summarized. Several modifications needed to make disc models more realistic are discussed. Finally, the extent to which these models are constrained by observations, such as detctions of “Soft X-Ray Excesses”, and prospects for obtaining future observational evidence of AGN accretion discs is considered.

UV excess

Almost from the first multi-frequency observations of Seyfert 1 nuclei and quasars, it was realized that their optical and ultraviolet spectra were far flatter than their infrared spectra, which have typical slopes of −1.2 (fv ∼ V−1.2). The different variability properties of the infrared and optical/ultraviolet continuum further suggest that they are produced by physically separated components (Cutri et al. 1985). The blue component (also known as the “UV excess” or “Big Blue Bump”) has a flux density rising with frequency in the optical, and a broad maximum somewhere in the ultraviolet. It falls (probably rather steeply) in the far- or extreme-UV. A falling high-frequency tail may be observed in the soft X-rays. This characteristic shape strongly suggests thermal emission from optically thick gas (Shields 1978, Malkan & Sargent 1982).