Extremophiles are organisms that are adapted to survive in extreme environments. This lecture describes the challenges that extremes of heat, cold, acidity, salinity, and radiation pose to organisms, and show examples of how evolution has nonetheless allowed some organisms to adapt to not just survive but thrive in such extreme conditions. Finally, we will explore the possible limits of life on Earth, and find that while you can make things pretty extreme and still have organisms adapt, you reach the limit if there is no water. Rather than being oddballs, these organisms give us important insights into the origins of life on Earth, and widens the possibilities for life on other worlds. Recorded live on 2009 Oct 16 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.