Information about the nature of the UV absorbing agents in Venus' atmosphere has been obtained from a comparison ofmodel predictions with a variety of photometric data obtained from the Pioneer Venus probes and orbiter as well as from the earth. The theoretical simulations were performed with numerically accurate radiative transfer codes that incorporated spacecraft constraints on the properties of aerosols and gases in Venus' atmosphere. We find that at least two UV absorbers are needed: gaseous SO2, which is a major source of opacity at wavelengths shortward of 0.32 &mgr;m, and a second absorber, which dominates above 0.32 &mgr;m. The concentration of the second absorber increases sharply with increasing optical depth within the first few optical depths and then has a relatiely flat profile throughout most of the remainder of the upper main cloud layer, but it is largely absent from the middle and lower cloud regions. Several mechanisms appear to be responsible for producing UV contrast features. These include changes in the optical depth of the upper haze layer, which leads to brightness differences between equatorial and polar regions, and changes in the depth over which the second UV absorber is depleted in the uppermost part of the main clouds, which creates UV features at equatorial latitudes. Atmospheric dynamics acts as a driving force for the above variations in the radiative properties of the clouds through dynamically induced temperature changes and changes in vertical mixing. Many UV-absorbing materials, including sulfur, various iron compounds, and nitrogen dioxide gas, have spectral absorption properties that are in serious disagreement with those deduced for the second UV absorber. one possibly tenable candidate for the second absorber is gaseous Cl2, whose spectral absorption characteristics closely match the empirically derived ones and whose expected altitude profile is crudely consistent with the sharp decreases in the concentration of the second absorber near the cloud tops and bottom of the upper clouds. Approximately 1 ppm of Cl2 is required. Whether this amount of Cl2 can be generated from the photodissociation of HCl is not clear, with the peak Cl2 mixing ratio being strongly dependent on the exact amount of HCl and other gases that are present in the upper clouds.