The Universe today is old, cold, low-density, and expanding. If we run the expansion backwards, we will eventually find a Universe where all the matter was in one place where the density and temperature are nearly infinite. We call this hot, dense initial state of the Universe the Big Bang. This lecture introduces the Big Bang model of the expanding universe, and how the history of the Universe depends on two numbers: the curretn expansion rate (H0), and the relative density of matter and energy (Omega0). Combined with observations, these give us an estimate of the age of the Universe of 14.0 +/- 1.4 Gyr. Recorded 2006 February 27 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.