A health alert is based on the validation of a signal indicating a threat to public health that requires immediate action. Environmental health warning signals have various sources: health and/or environmental surveillance systems, as well as local or societal sources. The purpose of this article is to examine the contributions, as well as the limitations and prospects, of environmental health surveillance systems in their role as warning mechanisms. If we define environmental health as human health in relation to the natural or anthropogenic environment, the main dedicated surveillance systems focus either on environmental factors (air and water pollutants, etc.) or environmental events (risk of death linked to a heat wave, etc.), or on health events (use of hospital emergency departments, Legionnaires’ disease, etc.). The continuity and adequate geographic coverage of data collection, the ability to rapidly access a critical mass of information to support decision-making, and a system for the rapid transmission of information to decision-makers and exposed populations are essential to ensure the responsiveness of the surveillance system and thus its early warning function. Furthermore, environmental health is characterized by the difficulty of identifying environmental exposures that pose a potential health risk, or of establishing a causal link between suspected environmental exposures and confirmed diseases. Therefore, opportunities for improvement in environmental health surveillance alert systems include increasing surveillance coverage, raising awareness among stakeholders, developing networks, and, complementarily, researching and utilizing new indicators and their health alert thresholds, as well as acquiring conclusive scientific knowledge on environment-related health effects. (R.A.)