This chapter explains how the emergent properties of water contribute to the suitability of the environment for life. Until fairly recently, scientists assumed that other physical requirements for life included a moderate range of temperature, pH, and atmospheric pressure. That view has changed with the discovery of organisms known as extremophiles, which have been found flourishing in hot, acidic sulfur springs and around hydrothermal vents deep in the ocean. What does the existence of life in such environments say about the possibility of life on other planets?