Life as known 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. This article discusses the following topics relating to astrobiology: what exactly it means to be alive; what astrobiologists do; extreme environments; and extreme environments on other planets.