What are the implications of Relativity for the Universe? This lecture introduces the Cosmological Principle, which states that the Universe is Homogeneous and Isotropic on Large Scales. Applying this to his then-new General Relativyt, Einstein got a surprise: the Universe must either expand or contract in response to all the matter/energy that fills it, something not observed in 1917. To attempt to stabilize the Universe, he introduced a Cosmological Constant (Lambda), that was to prove his greatest blunder. Subsequent theoretical and observational work was to establish that the Universe is indeed expanding systematically, if you look on scales large enough (the scale of galaxies). We will review observational evidence for the large-scale Homogeneity and Isotropy of the Universe, Einstein's brilliant conjecture, and see how the Cosmological Constant maybe wasn't such a blunder after all, as it has recently made a comeback of sorts. We'll explore these themes in greater detail in subsequent lectures. Recorded 2006 February 22 in 1008 Evans Laboratory on the Columbus campus of The Ohio State University.