What is the composition and structure of the Earth's atmosphere? Why is it as warm as it is, and how did it form? These are the questions for today's lecture. The Earth's atmosphere is a complex, dynamic, and evolving system. We will discuss the composition and structure of the atomsphere, the nature of the different thermal layers, the Greenhouse Effect, and the Primordial Atmosphere and atmospheric evolution. This will give us a basis for comparison when we begin to examine other planetary atmospheres in future lectures. Recorded 2006 Nov 1 in 100 Stillman Hall on the Columbus campus of The Ohio State University.