Life as people know it here on Earth exchanges energy and materials with the environment. Life forms grow, develop, produce waste products, and reproduce, storing genetic information in DNA and RNA and passing it from one generation to the next. Life evolves, adapting to changes in the environment and changing the environment in return. The basic unit of living things is the cell. Life is based on the chemistry of carbon and requires liquid water. Water has wonderful capabilities to dissolve the nutrients and salts on which life depends, and the ability to move the molecules into and out of cells as it flows. This article talks about astrobiology, the study of the origin, evolution, and distribution of life in the universe. It also discusses extreme environments on Earth and characterizes the life forms that occupy them. These environments provide Earthly analogues to environments on other planets, and a framework for thinking about the organisms that once did or may now inhabit them. The activities on the poster insert included with this issue are designed to explore this connection.