In this compelling book, leading scientists and historians explore the Drake Equation, which guides modern astrobiology's search for life beyond Earth. First used in 1961 as the organising framework for a conference in Green Bank, West Virginia, it uses seven factors to estimate the number of extraterrestrial civilisations in our galaxy. Using the equation primarily as a heuristic device, this engaging text examines the astronomical, biological, and cultural factors that determine the abundance or rarity of life beyond Earth and provides a thematic history of the search for extraterrestrial life. Logically structured to analyse each of the factors in turn, and offering commentary and critique of the equation as a whole, contemporary astrobiological research is placed in a historical context. Each factor is explored over two chapters, discussing the pre-conference thinking and a modern analysis, to enable postgraduates and researchers to better assess the assumptions that guide their research.