Why is the Earth habitable today but Venus and Mars not? This lecture explores the question of planetary habitability from the perspective of the stability of liquid water on the surface of planetary bodies. We will see how the amount of sunlight and the greenhouse effect in the atmosphere combine to create a classic Goldilocks problem: whether or not a planetary surface has stable liquid water is a question of not being too hot or too cold. This defines the Habitable Zone for the present-day Sun. However, the size of a planet also plays a role, and we will expand the concept of habitability to include the type of atmosphere a planetary body can or cannot retain. Finally, because the Sun changes brightness slowly over its lifetime, the location of the habitable zone is time-dependent. We will define the Continuous Habitable Zone, and discuss implications, and limitations, of the idea of habitable zones, looking forward expanding our search for life to worlds around other stars. Recorded live on 2009 Nov 5 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.