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The idea that there is life on other worlds has a long history that stretches back at least as far as the Greek philosophers’ writing in 400 BCE. More recently, Giordano Bruno (1548–1600 CE) formulated a cosmology that was remarkably ahead of his time. He subscribed to the Copernican heliocentric description of the solar system, considered that the stars were other suns, and would be orbited by planets just as the sun is. Moreover, he reasoned that there would be an infinite number of inhabited worlds. We had to wait almost 400 years after Bruno’s dreadful execution for the first exoplanet to be discovered. In the meantime, we have seen some fantastic claims, including life on the moon, and even the sun! For the first half of the twentieth century, the notion that Mars was inhabited was widely entertained; a view encouraged by illusory canals, surface features that darken and lighten with the Martian seasons attributed to the growth of vegetation and even, in 1956, the apparent detection of spectroscopic evidence for plant life. As a research enterprise, astrobiology—the study of the origin, nature, distribution, and future of life in the universe—is young. The term exobiology, the study of life elsewhere than on Earth, was first coined by the Nobel Prize-winning molecular biologist Joshua Lederberg, who became a powerful advocate for the new field. It emerged in the 1950s and 1960s from a convergence of planetary science, the search for exoplanets, origin of life studies, and the Search for Extraterrestrial Intelligence (SETI), four disciplines that use very different research methods and techniques and started out with very different goals. Unlike exobiology, described disparagingly in 1964 by an evolutionary biologist as a science without a subject, astrobiology—its successor—encompasses life on Earth. NASA has become its most important patron, spending $1 billion on the 1977 Viking missions to search for life on Mars, and funding much else, including biology programs. Astrobiology is a multidisciplinary pursuit that in various guises encompasses astronomy, chemistry, planetary and Earth sciences, and biology. It relies on mathematical, statistical, and computer modeling for theory, and space science, engineering, and computing to implement observational and experimental work. Consequently, when studying astrobiology we need a broad scientific canvas. For example, it is now clear that the Earth operates as a system; it is no longer appropriate to think in terms of geology, oceans, atmosphere, and life as being separate. This textbook reflects this multiscience approach, so if you are an astronomy major, there is some planetary science and biology to get to grips with, if you are a biologist, be prepared to learn some planetary science and astronomy, and if you are doing primarily Earth sciences— well, you get the picture.