Are variations in the energy generated by the Sun sufficient to modify the Earth’s global environment at levels comparable to expected anthropogenic changes? Debated contentiously for more than a century, this question must now be posed with new urgency: the proper specification of natural global changes is a prerequisite for detecting anthropogenic impacts. Important advances over the past decade in our knowledge of the Sun and of the terrestrial responses to solar variability provides the basis for answering this question with unprecedented surety, but significant uncertainties remain. This book addresses current monitoring and understanding of solar influences on both the climate system and the ozone layer and prioritizes the research effort that will be needed to provide a sound scientific basis for policymaking related to global change issues.