Androids are mechanical, or otherwise artificial, creations in the shape of humans. They have long been a staple of science fiction. From the clockwork persons of myth to Isaac Asimov's humanoid robots, to Star Wars's C-3PO, and to Steven Spielberg's A.I. Artificial Intelligence, imagined mechanical persons have enabled people to reflect upon what it means to be human.

The real world of androids is substantially more mundane than their appearance in science fiction. Although there exists a long history of clockwork automata and other mechanical imitations of persons, these have never been more than theatrical curiosities. The creation of more ambitious androids has had to await advances in robotics. Until the 1990s, the problems involved in creating a robot that could walk on two legs prevented robots from taking humanoid form. Yet if robotics technology continues to improve, then it seems likely that robots shaped like...