The supernova phenomenon was discovered because stellar explosions are prodigious sources of optical light. Although we know today that the energy budget for the core collapse of a massive star allocates 99% of the energy released to neutrinos and 1% to kinetic energy, the 10−4 part that becomes light makes super-novae the most luminous stars in the universe, and provides essential clues to the nature of supernova explosions. Even the pocket change of millionaires is significant: a typical supernova emits a substantial fraction of the luminosity of a typical galaxy, and the brightest have been detected a third of the way across the observable universe. This review summarizes recent work that uses the supernova light curve, the plot of luminosity versus time, to help derive a picture of supernova explosions and to explore the dimensions of the universe.