Astrobiology refers to the “origin, evolution, and distribution of life in the universe.” Astrobiologists study extreme environments on Earth and characterize 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. This article provides an overview of this fascinating field of study, and a wealth of content information as it relates to Earth and space science.