In contemporary times, policy makers and risk managers find themselves required to make decisions about how to prevent or mitigate complex risks that face society. Risks, such as global warming and energy production, are considered complex because they require knowledge from multiple scientific and technical disciplines to explain the mechanisms that cause and/or prevent hazards. This dissertation focuses on these types of situations: when experts from different disciplines and professions interact to coordinate and legitimize risk characterizations. A review of the risk communication literature highlights three main critiques: (1) Risk communication research historically treats expert groups as uniform and does not consider the processes by which they construct and legitimize risk understandings. (2) Risk communication research tends to privilege transmissive and message-centered approached to communication rather than examine the discursive management and coordination of different risk understandings. (3) Rather than assuming the taken-for-granted position that objective scientific knowledge is the source of legitimacy for technical risk understandings, risk communication research should examine the way that expert groups legitimate their knowledge claims and emphasize the transparency of norms and values in public discourse. This study performs an in-depth analysis of the case of cesium chloride. Cesium chloride is a radioactive source that has several beneficial uses medical, research, and radiation safety applications. However, it has also been identified as a security threat due to the severity of its consequences if used in a radiological dispersal device, better known as a “dirty bomb.” A recent National Academy of Sciences study recommended the replacement or elimination of cesium chloride sources. This case is relevant to the study of risk communication among multidisciplinary experts because it involves a wide variety of fields to discuss and compare terrorism risks and health risks. This study uses a multi-perspectival framework based on Bakhtin’s dialogism that enables entrance into the discourse of experts’ risk communication from different vantage points. Three main implications emerge from this study as seen through the lens of dialogism. (1) Expert risk communication in cross-disciplinary situations is a tension-filled process. (2) Experts who interact in cross-disciplinary situations manage the tension between discursive openness and closure through the use of shared resources between the interpretative repertoires, immersion and interaction with other perspectives, and the layering of risk logics with structural resources. (3) The emergence of security risk Discourse in a post-9/11 world involves a different set of resources and strategies that risk communication studies need to address. In the case of cesium chloride issue, the interaction of experts negotiated conflict about the characterization of this isotope as a security threat or as being useful and unique. Even though participants and organizations vary in how they characterize cesium chloride, most maintained some level of balance between both characterizations—a balance that was constructed through their interactions with each other. This project demonstrates that risk characterizations risks shape organizational decisions and priorities in both policy-making and regulatory organizations and private-sector and functional organizations.