An early warning system is a warning system that can be implemented as a chain of information communication systems and comprises sensors, event detection and decision subsystems for early identification of hazards. They work together to forecast and signal disturbances that adversely affect the stability of the physical world, providing time for the response system to prepare for the adverse event and to minimize its impact.

To be effective, early warning systems need to actively involve the communities at risk, facilitate public education and awareness of risks, effectively disseminate alerts, and warnings and ensure there is constant state of preparedness. A complete and effective early warning system supports four main functions: risk analysis, monitoring and warning; dissemination and communication; and a response capability.

Risk analysis involves systematically collecting data and undertaking risk assessments of predefined hazards and vulnerabilities. Monitoring and warning involves a study of the factors that indicate a disaster is imminent, as well as the methods used to detect these factors. Dissemination and communication concerns communicating the risk information and warnings to reach those in danger in a way that is clear and understandable. Finally, an adequate response capability requires the building of national and community response plan, testing of the plan, and the promotion of readiness to ensure that people know how to respond to warnings.

An early warning system is more than a warning system, which is simply a means by which an alert can be disseminated to the public.

Early-warning radars, early warning satellites, and Airborne early warning and control are systems used for detecting potential missile attacks. Throughout human history the warning systems that use such have malfunctioned several times, including some nuclear-weapons-related false alarms. Due to the massive availability of information through (social) media, early-warning systems that use these vast amounts of information are also developed to potentially detect risks of terrorism and novel terror attacks. This builds on the assumption that aggregated news coverage functions as a wisdom-of-the-crowd mechanism, where aggregated (and quantified) information can provide a reliable and cost-effective source of information for more accurate and precise predictions.

The easiest or most likely artificial signals from Earth to be detectable from around distant stars are brief pulses transmitted by such anti-ballistic missile (ABM) early-warning and space-surveillance radars during the Cold War and later astronomical and military radars.

Scientists are researching and developing systems to predict eruptions of volcanoes, earthquakes and other natural disasters.

Early warning systems could be developed and used to prevent and mitigate pandemics, e.g. before they spillover from other animals to humans, and disease outbreaks.