Is there any evidence that the Universe was very hot and dense in the distant past as predicted by the Big Bang model of the expanding Universe? This lecture examines observational tests of the Big Bang Model. We have already covered expansion in the previous lecture. Today we look at Primordial Nucleosynthesis, the creation of light elements from fusion during the first 3-4 minutes of the hot phases of the Big Bang, and the Cosmic Background Radiation, the relic blackbody radiation remaining from when the Universe became transparent to light 300,000 years after the Big Bang. Both predictions of the Big Bang Model have been spectacularly confirmed by observations of the present-day Universe. These give us confidence that the Big Bang, in broad outline, is the correct physical model of our expanding Universe. Recorded 2006 February 28 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.