As a key player in public health alert systems in France, the French Institute for Public Health Surveillance (InVS) compiled a list of 106 environmental threats to public health in 2009. Building on this inventory, the objective of this study was to identify existing environmental health alert systems, describe their purpose, structure, and organizational arrangements, and, finally, assess their value for stakeholders. Based on a review of the websites of the main countries active in this field and interviews with key stakeholders (industry representatives, government agencies, and associations), the study facilitated a discussion to clarify the often multifaceted concepts of threat, surveillance, and alert. It offers insights into the similarities among surveillance and alert systems developed worldwide and produces a classification through the modeling of these systems, which is useful for understanding the challenges of public policy. Drawing on control theory from engineering sciences, these systems can be modeled as loops that regulate the state of the environment and/or health through control mechanisms fed by measurements of environmental and/or health indicators. This generic representation is complemented by the concept of temporality, which distinguishes between immediate and delayed responses, as well as between immediate variability and the delayed response to the event. Interpreting the results of the monitoring system is not straightforward, particularly due to the difficulty in identifying the fraction of risk attributable to one or more specific agents. By distinguishing between known and unknown threats and the chronic or acute nature of exposure, this theoretical model allows for a better understanding of how to monitor environmental health and design alert systems to facilitate action. Three categories are identified: environmental monitoring for health purposes, specific health monitoring in environmental health, and generic health monitoring. Two perspectives can be discussed. The first is based on an integrative approach to environmental and health indicators for precise risk identification. The second aims to prioritize exposure situations by risk level. (R.A.)