The idea of using radio telescopes to search for alien messages, or merely to eavesdrop on extraterrestrial radio traffic, dates back to the 1920s. The power and sensitivity of radio, together with the existence of an electromagnetic "window" in our atmosphere at these frequencies, suggest that this is an appropriate technology for such a search.
An early pioneer of radio SETI was the American astronomer Frank Drake. He wrote down a famous formula -- the Drake equation -- that estimates the number of technological civilizations that might exist in our galaxy. Each term in the equation represents the probability of some key step in the evolution of such a civilization, and the numbers involved are highly conjectural. These terms are: the average rate of star formation, the fraction of stars that are stable and long-lived, the fraction of such "good" stars that have planets, the likely number of such planets that are "Earth-like," the fraction of Earth-like planets on which life will develop, the fraction of such biospheres that develop intelligence, the fraction of intelligent species that develop technology and, finally, the average lifetime of a technological community. Multiplying all these factors yields a guess for the expected number of communicable civilizations in our galaxy at this time.