This study by an eminent scientist, a former U.S. arms control official and a specialist on Soviet defense policy at Stanford, is strongly critical of President Reagan's Strategic Defense Initiative, popularly known as "Star Wars," whether it is viewed as transcending deterrence through a nearly foolproof defense or, as more recently argued, as enhancing deterrence through a partial defense. The authors review the proposed technologies, as well as the countermeasures by which offensive systems could offset defensive systems, and question the technological feasibility and the cost assumptions. They confirm the findings of other reports. But the principal value of this one is in its discussion of the political effects upon the Soviet Union, U.S. allies, and arms control negotiations. Strategic stability would be impaired, it is argued, if in the absence of a major new arms control agreement both sides built up their offensive forces while simultaneously developing their new defense capabilities. The impact upon European and Asian allies would be decoupling, pushing them into new nuclear programs and to questioning American guarantees. One need not agree with the conclusions of this study to recognize its merits. A most useful contribution to what will surely be a growing debate.