In 1964, Sjur Refsdal first considered the possibility that the light from a background supernova could traverse multiple paths around a strong gravitational lens towards us. He showed that the arrival times of the supernova’s light would depend on the cosmic expansion rate, as well as the distribution of matter in the lens. I will discuss relative time-delay and magnification measurements of the first such multiply imaged supernova, which exploded behind the MACS J1149.6+2223 galaxy cluster. We have developed a new Hubble Space Telescope photometry pipeline, and applied multiple light-curve fitting codes coupled with detailed simulations to reach optimal inferences from the SN’s light curve. The reappearance of the SN in 2015 enables a novel constraint on the Hubble constant, and new, blind tests of galaxy-cluster lens models.