Computers more intelligent than humans? Self-replicating molecular robots? Virtual immortality? These may sound like science fiction, but some reputable computer scientists are predicting they will happen within the next several decades. What will our world be like if and when our machines surpass us in intelligence? Do the advances in biotechnology, robotics, and nanotechnology, which make intelligent machines possible, pose dangers of their own? Should we embrace such a future or try to stop it?