The web of measures that comprise the nuclear non-proliferation regime continues to hold at bay the "nuclear-armed crowd" that was part of President John F. Kennedy's alarming vision in 1963. The number of nuclear weapons states in 2004 stands at only eight or nine, and assertive steps may yet keep this number from growing. The proliferation of biological weapons, however, is quite another matter. Biotechnological capacity is increasing and spreading rapidly. This trend seems unstoppable, since the economic, medical, and food-security benefits of genetic manipulation appear so great. As a consequence, thresholds for the artificial enhancement or creation of dangerous pathogens--disease-causing organisms--will steadily drop. Neither Cold War bilateral arms control nor multilateral non-proliferation provide good models for how we are to manage this new challenge. Much more than in the nuclear case, civilization will have to cope with, rather than shape, its biological future.