The use of thermonuclear supernova explosions as standard candles led to the discovery of cosmic acceleration and the search for the nature of dark energy. How good are these standard candles? How do we understand and model the variation in supernova properties? How does all of this fit in with what we know about stellar evolution and cosmic enrichment? I’ll describe what we know (and don’t know) about these questions and what is being done to answer them more securely in advance of a possible Joint Dark Energy Mission to measure the properties of dark energy.