Recent measurements made from platforms in space prove beyond question that the radiant energy received from the Sun at the Earth, once called the ‘solar constant’, fluctuates over a wide range of amplitudes and time scales. The source of that variability and its impact on our terrestrial environment pose major challenges for modern science. We are confronted with a tangled web of facts which requires the combined ingenuity of solar, stellar, planetary and atmospheric scientists to unravel. This brief overview draws attention to key developments during the past century which shaped our concepts about sources of solar variability and their connection with solar activity.