In the past few decades the human species has begun, seriously and systematically, to look for evidence of life elsewhere. While no one has yet found living organisms beyond the earth, there are some reasons to be encouraged. Robotic space probes have identified worlds where life may once have gained a toehold, even if it does not flourish there today. The Galileo spacecraft found clear signs of life during its recent flight past the earth—a reassurance that we really do know how to sniff out at least certain kinds of life. And rapidly accumulating evidence strongly suggests that the universe abounds with planetary systems something like our own.