This paper discusses the nature and causes of strategic stability and instability, how defenses affect the stability equation, problems of partial defenses and various attempts to model stability during the transition to effective defense. All of the models reviewed demonstrate that there is some stable path from no defenses to near perfect (99 percent effective) defenses, and the paper presents an integrated theory that shows the common nature of all of the stable paths. This paper demonstrates that the results of the various transition stability differ not in numerical results but in the measures of stability used. Recommendations for future transition stability studies are included, based on the review of the various models. The appendices contain detailed descriptions of models used in the various transition stability studies reviewed.