In 1961 Frank Drake wrote down his now-eponymous equation with which one can predict the number of technological civilizations in the Milky Way galaxy.

If one can determine the values of the various factors of the equation, then the Drake equation provides an estimate of the typical number of technological civilizations in the galaxy at any given time. Unfortunately, unlike most other equations in physics and astronomy, not only are the values of the various factors of the equation not well determined, it will probably be some time before they are. SETI researchers therefore approach the Drake equation from the other direction. They attempt to count the number of technological civilizations in the galaxy in the hopes that the number (or absence!) of such civilizations will tell us something about the various factors in the Drake equation.

Astronomers seek evidence of three categories of extraterrestrial civilization. As used by Horowitz and Sagan, a Type 0 civilization is one that uses 10 terawatts of power--i.e., the civilization on Earth today. A Type I civilization uses the total solar power falling on its planet, about 10,000 times more than a Type 0. A Type II civilization uses the full luminosity of its star, for transmissions about 1 billion times more power than a Type I. --Joseph Lazio