In June 1985, a software engineer named David Parnas resigned from a panel established to evaluate the computer software used by U.S. President Ronald Reagan’s antimissile Strategic Defense Initiative. Parnas objected to Reagan’s “Star Wars” system because there was no sure way to remove the risk of a catastrophic failure, since there would be no way to find bugs in the system before it faced its first and possibly only great test. Why, asks Slayton, did it take so long for dilemmas like that one to take center stage in the debate over missile defense? She provides the answer in an original and engrossing study. Since the earliest days of thinking about missile defense, in the 1950s, the field had been dominated by physicists who were mostly preoccupied with the problem of identifying and tracking incoming warheads. Only when software specialists began to acquire more professional standing did their concerns come to the fore. In addition to providing new insights into the debate over missile defense, Slayton raises valuable questions about the broader interaction between scientific expertise and advocacy.