On April 8, 1960, a young American astronomer, Frank Drake, turned a radio telescope toward the star Tau Ceti and listened for several hours to see if he could detect any artificial radio signals. With this modest start began a worldwide project of potentially momentous significance. Known as SETI - Search for Extraterrestrial Intelligence - it is an amalgam of science, technology, adventure, curiosity and a bold vision of humanity's destiny. Drake has said that SETI is really a search for ourselves - who we are and what our place might be in the grand cosmic scheme of things. Yet with one tantalizing exception, SETI has produced only negative results. After millions of hours spent eavesdropping on the cosmos astronomers have detected only the eerie sound of silence. What does that mean? Are we in fact alone in the vastness of the universe? Is ET out there, but not sending any messages our way? Might we be surrounded by messages we simply don't recognize? Is SETI a waste of time and money, or should we press ahead with new and more sensitive antennas? Or look somewhere else? And if a signal were to be received, what then? How would we - or even should we - respond