The distinguished international relations theorist Kenneth Waltz (still) thinks that the emergence of new nuclear powers is not necessarily to be regretted. In a world of many nuclear powers, in Waltz's view, a major war would be practically impossible. He therefore believes that "more may be better"-that what has come to be called "nuclear proliferation" might actually be a good thing. Scott Sagan, a professor of political science at Stanford University and a leading authority in the field of strategic studies, disagrees; in Sagan's view, "more will be worse." In 1995, Waltz and Sagan published The Spread of Nuclear Weapons: A Debate, in which each laid out his argument and then responded to the other's argument. The discussion was spirited and often quite interesting, and the book deservedly received a good deal of attention. A revised edition, with new passages on terrorism and missile defense and a new chapter on India and Pakistan, has just been published.