Spectral measurements of solar ultraviolet (UV) radiation have been made at several ground-based locations and for more than 10 yr at some sites. These measurements are important for two main reasons. First, the measurements combined with results of radiative transfer models contribute toward our understanding of the many complicated radiative transfer processes in the atmosphere and at the Earth's surface. These processes include absorption of radiation by atmospheric gases such as ozone and sulfur dioxide, scattering by atmospheric aerosols and clouds, and scattering from the earth's surface. Knowledge of these processes is required for operational applications such as the estimation of surface UV radiation from satellite data and the forecasting of the UV index. Also, our ability to estimate UV climatology in the past, as well as in the future, requires thorough knowledge of the UV radiative transfer processes. The second reason for making systematic ground-based measurements of UV radiation is to determine whether long-term changes are occurring as a result of ozone depletion or climate change and to identify specific causes. Examples of how long-term ground-based data records have contributed to our understanding of surface UV radiation are presented.