The proper study of mankind is not merely Man, but Intelligence. 1 —Arthur C. Clarke, 1951 In the long-running television series "The X-Files," the original Deep Throat said to FBI Special Agent Mulder that "there are those like yourself who believe in the existence of extraterrestrial life." Ah, but that's not the question. If extraterrestrial life exists, most of it may be in simpler forms comparable to the one-celled organisms of Earth biology. Finding such life would be fascinating for scientists, but may be of only passing interest to the general public. What intrigues the average citizen is the possibility of contact with extraterrestrial intelligence. We want to communicate with other sentient beings, learning what they know and telling them about ourselves. We want to ? nd out how they are like us and how they are different. Microorganisms don't have a lot to say. There is another implication of contact that underlies this book: Intel- gent extraterrestrials might have an impact on our future. The information they send us—if any—might change our cultures. They could have c- scious intentions toward us, and possibly the technologies to reach us directly. Their intentions may be benign—or not. Our interest in alien minds is not new. The idea that intelligent beings exist beyond the Earth has been part of the Western intellectual tradition for more than 2000 years. Sometimes this belief was widespread; at other times, it was out of fashion.