The world has observed an increase in the number of nuclear powers. What would be the consequences of the nuclear proliferation in the twenty-first century? Would the new century be more unstable or not? The Cold War had been quite stable for the “balance of terror” which was the unintended consequence of the nuclear weapons. In contrast, the nuclear proliferation in the new century would not follow the footsteps of the Cold War; instead, it would bring more dangers than benefits. Emerging proliferators such as Pakistan, North Korea, and Iran would have difficulties in building credible deterrent capability for their political setbacks. They might sell the nuclear devices to terror groups, contributing to nuclear terrorism. There would be accidents with nuclear explosions. Their emergence into nuclear status could trigger nuclear arms races particularly in Asia. It is of great importance to stop the current trend to proliferation, but it is more critical to contain destabilizing effects from the proliferation and to encourage the proliferators to build stability-friendly nuclear forces.