This chapter contains sections titled: A Brief History of the Assessment of Habitability What Determines Habitability? Uninhabited Habitable Worlds Factors Determining Habitability Habitability and Temperature Habitability and Energy Other Factors that Determine Habitability A Postulate for Habitability Assumptions about the Habitat Assumptions on Life Attempt to Formulate a Habitability Postulate Some Test Cases for Habitability Test Case One: Life on Venus Test Case Two: Life on the Early Earth Résumé of the Two Test Cases Conclusions Further Reading Books and Articles in Books Articles in Journals Questions for Students