At the outer limits of the range of topics covered in these proceedings from a joint meeting of the British Royal Society and the French Academie des Sciences lie speculation and inaccessible places and times. The causes of the Sun's variability are hidden beneath its surface, and we can only speculate about whether solar magnetism, which provides the energy for the variations, is generated just below the surface or is a remnant of an ancient magnetism trapped in the core. An equally distant and inaccessible sphere is the gassy envelope around the prehistoric Earth where changes in climate presumably drove early man to alter his habits and homes. But as P. I. Kuniholm writes in this volume, climate change has often been appealed to in accounting for phenomena that cannot otherwise be accounted for. Archaeologists never imagined that physicists would try to reconstruct the Sun's past from such speculations. Between the two extremes of ignorance, however, a solid foundation is being built for a scientific account of the Sun's role in climate change.