Abstract
Having laid some extensive groundwork, we are finally ready to address the central questions of the existence and possible nature of extraterrestrial intelligent life. Although no trace of such life has so far been found, there are clearly compelling reasons for its existence. In Chap. 4 we estimated that Earth-like planets should occur quite frequently in our galaxy, and in Chaps. 5 and 6 it was shown how life, and intelligent life, have formed on Earth. Using the Drake formula, these results will now be combined to estimate the expected number of extraterrestrial intelligent societies. Since many probability factors entering this equation are controversial, the opinions of various authors will be discussed.
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