Abstract: What is causing the recent acceleration of the universe? A new component to the energy or a departure from our theory of gravity? Either way, observations can probe the physics of this "dark energy." Combined with cosmic microwave background observations, data on the position and apparent shapes of billions of distant galaxies will provide several independent probes of dark energy. I will describe an experiment, called LSST, which will undertake these multiple high- precision probes using a single wide-fast-deep survey of half the sky.