What is the composition and structure of the Earth's atmosphere? Why is it as warm as it is, and how did it form? Today I will describe the composition and structure of the atmosphere, the Greenhouse Effect, the Primordial Atmosphere, and Atmospheric Evolution. The Earth's atmosphere is a complex, dynamic, and evolving system, and we will use it as a point of comparison when we begin to examine other planetary atmospheres in future lectures. Recorded 2007 Oct 31 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.