Variations in the activity of the Sun have long been suspected to affect the climate of the Earth, and a number of correlations have been presented that indicate a link. In many cases, the presented correlations have been associated with poor statistical significance. Recent results, however, have indicated strong correlations between climate parameters and solar activity. Upper troposphere and stratosphere temperatures have been found to vary in phase with the 10- to 12-year solar activity cycle. On a longer time scale, the global temperature, particularly the Northern Hemisphere land air temperature, has been found to be nearly perfectly correlated with the long-term variation of solar activity. The solar activity itself cannot be represented by only one parameter. Different representations of solar activity have different and not understood, long-term variations. It has been found that the longterm variation of solar activity represented by the varying length of the approximately ii-year solar cycle is the parameter that is probably best correlated with global temperature, both with respect to the modern instrumental temperature record from 1860 to 1990 and with respect to a reconstructed temperature record extending back to 1750 when sunspot observations are believed to have been reliable.