Louisiana’s chemical corridor (United States) and northern Italy’s Porto Marghera chemical region are both sites of long-term, highly visible citizen struggles. In both locations the debates about, and the shaping of, environmental health knowledge related to toxicants was key to the emergence of the controversy as well as its outcome. This chapter examines these dynamics, particularly those of citizen-expert alliances, to develop an understanding of the construction of policy-relevant or “actionable” science. Specifically, my research focuses on the intersection of citizen activism, environmental health science, the public use of science, industrial regulation, and policy change related to toxicants in petrochemical