Everything we know about how planets form and how life arises suggests that human civilization on Earth should not be unique. We ought to see abundant evidence of extraterrestrial activity--but we don't. Where is everybody? In this volume in the MIT Press Essential Knowledge series, science and technology writer Wade Roush examines one of the great unsolved problems in science: is there life, intelligent or otherwise, on other planets? This paradox (they're bound to be out there; but where are they?), first formulated by the famed physicist Enrico Fermi, has fueled decades of debate, speculation, and, lately, some actual science. Roush lays out the problem in its historical and modern-day context and summarizes the latest thinking among astronomers and astrobiologists. He describes the long history of speculation about aliens; the emergence of SETI (the Search for Extraterrestrial Intelligence) as a scientific discipline in the 1960s, and scientists' use of radio and optical techniques to scan for signals; and developments in astrobiology and exoplanet research. Finally, he discusses possible solutions to the Fermi Paradox and suggests way to refocus SETI work that might increase the chances of resolving the paradox--and finding extraterrestrials.