The Sun changes its level of activity periodically at many different frequencies. These cycles can last from a solar rotation (27 days) to a century or more. The most well-known activity cycle is the so-called sunspot cycle, whose period lasts about 11 years. The total solar irradiance changes only little at different levels of solar activity, but its UV part varies much more. Here we will give an overview of the changes of the solar UV irradiances due to changes of solar activity. Furthermore we will briefly review their possible contribution to climate change, as proposed by several authors.