If we were ever to detect an alien transmission from outer space, ought we to reply? There are good grounds for keeping silent. After all, how do we know that any such aliens will be peaceable? But the truth is that it is too late for such precautions. Since World War II, we've been broadcasting TV, radio, and radar inadvertently into the heavens. Admittedly, these signals would be very weak at light years' distance. But the difficulty extraterrestrials would have in picking these up is nothing compared to the challenge they'd face in launching weaponry our way. Any society with the capability to threaten Earth will certainly already have the kit required to pick up the leakage we've been wafting skyward for seven decades.