The effect that defenses have on crisis stability is one of the central issues in the current debate over the Strategic Defense Initiative (SDI). Crisis stability would be threatened in a situation in which either the United States or the Soviet Union feels pressure to strike first in the hope of producing a more favorable outcome to what appears to be inevitable nuclear war. This Note examines various issues surrounding the question of crisis stability. In particular, it considers options for ameliorating any potential instabilities that might arise during a transition to higher levels of strategic defense. The authors suggest that a crisis-stable transition is possible, that the mutual survivability of strategic offensive forces is crucial to maintaining crisis stability, that space-based defenses of strategic offensive forces (SOF) are potentially crisis destabilizing unless they are backed up by terminal defenses of SOF, and that the mutual survivability of strategic defenses and defense suppression forces is also critical to crisis stability.