The past ten years has seen progress in the understanding of the ultraviolet background of the earth's atmosphere. These airglow, auroral, and scattering emission sources set limits on the usefulness of UV for space observations. They also, however, enable several types of passive remote sensing; such as, electron density profiles, neutral density and composition, and auroral location and strength. The paper describes our measurements and data analysis in these areas. UV imaging of the aurora has been achieved by four experiments, and these are briefly reviewed. UV imaging has opened up a new way to study solar-terrestrial relationships.