Our symposium answers the ICHSTM call for studies of “knowledge at work” by examining hygiene’s essentially applied-and urban-character. The modern city was both the social context that spawned the science and its main object of study and field of application. At the intersection of urban studies and histories of science and medicine, a significant body of research now examines the reciprocal impact of science and urban environments (see esp. Science in the City, Osiris vol. 18 [2003], focused largely on Paris). We trace hygiene’s concrete application in nineteenth- and twentieth-century Paris, asking how Paris’s local history of hygienic intervention into the built environment and daily practices supports or contradicts its oft-noted urban identity as a global “crucible” and “capital of reference” for hygienic science (Chevallier 2010). Treating topics from physical fitness and food to death, medicine, noise pollution, and international expositions, our six papers track hygiene through studies of architecture and urban planning, urban ecology, medicine, the body and its senses, and transnational urban networks. Only this interdisciplinary approach can capture the manifold character of urban hygiene, which itself was a sprawling, interdisciplinary field that ranged far beyond medicine and public health. Finally, we aim to continue bilingual dialog among scholars of France in Europe and North America.