In the 20th century's final decades, expert judgment in the form of quantitative risk assessment became the authoritative mode for understanding and expressing health and technological threats in the United States. By the mid-1980s, indeed, risk assessment had become one of the fundamental bases for public policy and federal regulation. Although the approach was new, controversial, and plagued by uncertainty, risk assessment was applied broadly as a means for giving justifiable, "scientific" answers to difficult political questions raised by modern science and technology. The professionalization of risk assessment in the 1970s and 1980s, which this dissertation places in historical context, was an important signpost for broader shifts in American approaches to risk during the last half of the 20th century. Along with the related fields of risk management and risk communication, formal risk assessment codified an increasingly authoritative expert epistemology based on distanced, statistical calculations. To illustrate these developments, this dissertation analyzes two case studies involving exposures to radon gas, an occupational and residential carcinogen and a hazard that the federal government attempted to manage both before and after the professionalization of risk assessment. These radon crises---the first in uranium mines in the Southwest in the 1950s and 1960s, and the second in suburban homes in the 1980s---are a lens through which the dissertation examines the ongoing tension between an expert epistemology that relied on quantification and a non-expert epistemology based on local knowledge, empirical evidence, and personal experience. These cases illustrate ongoing attempts to construct ideal citizens through risk communication and the media, and they reveal changes and continuities in the American public's perception of risk during the second half of the 20th century. This dissertation historicizes risk and the discourses around it, showing how regulatory agencies, the media, risk professionals, and private citizens employed narratives about risk to support their own epistemological systems, make claims to power or autonomy, and demarcate a shifting relationship between the government and its citizens.