A historic puzzle concerned the source of energy that can maintain the optical supernova glow for months. Although the energy that disrupts each type of supernovae is delivered promptly, the light curves are dominated by subsequent radioactive heating of the rapidly expanding ejecta. Some have considered rotational energy from the central pulsar. The ejecta gases would dim quickly without some energy input to keep it hot. The intensely radioactive nature of the ejecta gases, which is now known to be correct for most supernovae, was first calculated on sound nucleosynthesis grounds in the late 1960s. It was not until SN 1987A that direct observation of gamma-ray lines unambiguously identified the major radioactive nuclei.