The characteristics of the Île-de-France region, particularly its high level of urbanization, give rise to nuisances—notably noise and pollution—and raise questions about their health effects. It is therefore essential to have access to ongoing information regarding both exposure levels and their nature, as well as their impact on the quality of life and health of residents in the Île-de-France region. The objective is to demonstrate how two “generalist” health surveillance organizations, such as the ORS (Île-de-France Regional Health Observatory) and the Cire (Interregional Epidemiology Unit), have successfully developed complementary and specific regional expertise in the field of monitoring environmental health effects. The origins and history of these organizations, along with an analysis of their activities, reveal a gradual expansion of their expertise across complementary timeframes and fields. The Cire examines specific, localized, “real-time” environmental health issues, while the ORS establishes the broader framework, assesses their significance within the region, and tracks their long-term trends. The still-partial structuring of environmental health monitoring and surveillance in the region must continue, particularly by strengthening and formalizing the complementary roles of these two organizations. (R.A.)