The Emergency Alert System (EAS) is a national warning system in the United States put into place on January 1, 1997 (approved by the Federal Communications Commission (FCC) in November 1994), when it replaced the Emergency Broadcast System (EBS), which in turn replaced CONELRAD. It is jointly coordinated by the Federal Emergency Management Agency (FEMA), the Federal Communications Commission (FCC) and the National Oceanic and Atmospheric Administration (NOAA). The EAS regulations and standards are governed by the Public Safety and Homeland Security Bureau of the FCC.

As with its predecessors, the system is primarily designed to allow the president to address the country via all radio and television stations, in the event of a national emergency. Despite this, neither the system nor its predecessors have been used in this manner, due to the ubiquity of news coverage in these situations. In practice, it is more commonly used to distribute information regarding imminent threats to public safety, such as severe weather situations (including flash floods and tornadoes), AMBER Alerts of child abductions, and other civil emergencies.

Authorized organizations are able to disseminate and coordinate emergency alerts and warning messages through EAS. The Integrated Public Alert and Warning System (IPAWS) is used as a backend to distribute alert information via EAS and related technologies such as Wireless Emergency Alerts (WEA), using Common Alerting Protocol (CAP).EAS messages are transmitted primarily via terrestrial and satellite radio and television (including broadcast and multichannel television), which are required to participate in the system.

Messages in the EAS are composed of four parts: a digitally encoded Specific Area Message Encoding (SAME) header, an attention signal, an audio announcement, and a digitally encoded end-of-message marker. The About this soundSAME header (help·info) is the most critical part of the EAS design. It contains information about who originated the alert (the president, state or local authorities, the National Weather Service (NOAA/NWS), or the broadcaster), a short, general description of the event (tornado, flood, severe thunderstorm), the areas affected (up to 32 counties or states), the expected duration of the event (in minutes), the date and time it was issued (in UTC), and an identification of the originating station (see SAME for a complete breakdown of the header).

There are 77 radio stations designated as National Primary Stations in the Primary Entry Point (PEP) System to distribute presidential messages to other broadcast stations and cable systems. The Emergency Action Notification is the notice to broadcasters that the president of the United States or their designee will deliver a message over the EAS via the PEP system. The government has stated that the system would allow a president to speak during a national emergency within 10 minutes.

The National Public Warning System, also known as the Primary Entry Point (PEP) stations, are a network of 77 radio stations that are, in coordination with FEMA, used to originate emergency alert and warning information to the public before, during, and after incidents and disasters. PEP stations are equipped with additional and backup communications equipment and power generators designed to enable them to continue broadcasting information to the public during and after an event. Beginning with WJR/Detroit and WLW/Cincinnati in 2016, FEMA began the process of constructing transportable studio shelters at the transmitters of 33 PEP stations, which feature broadcasting equipment, emergency provisions, a rest area, and an air filtration system. NPWS project manager Manny Centeno explained that these shelters were designed to "[expand] the survivability of these stations to include an all hazards platform, which means chemical, biological, radiological air protection and protection from electromagnetic pulse."

The FEMA National Radio System (FNARS) "Provides Primary Entry Point service to the Emergency Alert System", and acts as an emergency presidential link into the EAS. The FNARS net control station is located at the Mount Weather Emergency Operations Center.Once an EAN is received by an EAS participant from a PEP station (or any other participant) the message then "daisy chains'" through the network of participants. "Daisy chains" form when one station receives a message from multiple other stations and the station then forwards that message to multiple other stations. This process creates many redundant paths through which the message may flow increasing the likelihood that the message will be received by all participants and adding to the survivability of the system.Each EAS participant is required to monitor at least two other participants.

Because the header lacks error detection codes, it is repeated three times for redundancy. EAS decoders compare the received headers against one another, looking for an exact match between any two, eliminating most errors which can cause an activation to fail. The decoder then decides whether to ignore the message or to relay it on the air if the message applies to the local area served by the station (following parameters set by the broadcaster).The SAME header bursts are followed by an attention tone, which lasts between 8 and 25 seconds, depending on the originating station. The tone is About this sound1050 Hz (help·info) on a NOAA Weather Radio station. On commercial broadcast stations, a About this sound"two-tone" (help·info) attention signal of 853 Hz and 960 Hz sine waves is used instead, the same signal used by the older Emergency Broadcast System. These tones have become infamous, and can be considered both frightening and annoying by viewers; in fact, the two tones, which form approximately the interval of a just major second at an unusually high pitch, were chosen specifically for their ability to draw attention, due to their unpleasantness on the human ear. The SAME header is equally known for its shrillness, which many have found to be startling. The "two-tone" system is no longer required as of 1998, and is to be used only for audio alerts before EAS messages.[full citation needed] Like the EBS, the attention signal is followed by a voice message describing the details of the alert.The mes sage ends with 3 bursts of the AFSK "EOM", or End of Message, which is the text NNNN, preceded each time by the binary 10101011 calibration.

Under a 2006 executive order issued by George W. Bush, the U.S. government was instructed to create "an effective, reliable, integrated, flexible, and comprehensive" public warning system. This was accomplished via expansions to the aforementioned PEP network, and the development of the Integrated Public Alert and Warning System (IPAWS)—a national aggregator and distributor of alert information using the XML-based Common Alerting Protocol (CAP) and an internet network. IPAWS can be used to distribute alert information to EAS participants, supported mobile phones (Wireless Emergency Alerts), and other platforms.Under an FCC report and order issued in 2007, EAS participants would be required to migrate to digital equipment supporting CAP within 180 days of the specification's adoption by FEMA. This officially occurred September 30, 2010, but the deadline was later delayed to June 30, 2012 at the request of broadcasters. The FCC has established that IPAWS is not a full substitute for the existing SAME protocol, as it is vulnerable to situations that may make internet connectivity unavailable. Therefore, broadcasters must convert CAP messages to legacy SAME headers to enable backwards compatibility with the existing "daisy chain" method of EAS distribution, providing a backup distribution path.

The FCC requires all broadcast stations and multichannel video programming distributors (MVPD), hereafter "EAS participants", to install and maintain FCC-certified EAS decoders and encoders at their control points or headends. These decoders continuously monitor the signals from other nearby broadcast stations for EAS messages. For reliability, at least two source stations must be monitored, one of which must be a designated local primary. Participants are to retain the latest version of the EAS handbook.EAS participants are required by federal law to relay Emergency Action Notification (EAN) messages immediately (47 CFR Part 11.54).[16] Broadcasters traditionally have been allowed to opt out of relaying other alerts such as severe weather, and child abduction emergencies (AMBER Alerts) if they so choose (in practice, severe weather coverage is usually handled by a station's local news operation and on-air meteorologists, rather than processed through EAS). EAS participants are required to keep logs of all received messages. Logs may be kept by hand but are usually kept automatically by a small receipt printer in the encoder/decoder unit. Logs may also be kept electronically inside the unit as long as there is access to an external printer or method to transfer them to a computer.In addition to the audio messages transmitted by radio stations, television stations must broadcast a visual display containing the originator, event, location, and time period of the alert, as well as any extended text that is contained within the associated CAP message.

All EAS equipment must be tested on a weekly basis. The required weekly test (RWT) consists, at a minimum, of the header and end-of-message tones. Though an RWT does not need an audio or graphic message announcing the test, many stations provide them as a courtesy to the public. In addition, television stations are not required to transmit a video message for weekly tests. RWTs are scheduled by the station on random days and times, (though quite often during late night or early afternoon hours), and are generally not relayed.A Required Monthly Test (RMT) transmitted in New Jersey on April 15, 2014 as shown on a television set Required monthly tests (RMTs) are generally originated by the local or state primary station, a state emergency management agency, or by the National Weather Service and are then relayed by broadcast stations and cable channels. RMTs must be performed between 8:30 a.m. and local sunset during odd numbered months, and between local sunset and 8:30 a.m. during even numbered months. Received monthly tests must be retransmitted within 60 minutes of receipt. Additionally, an RMT should not be scheduled or conducted during an event of great importance such as a pre-announced presidential speech, coverage of a national/local election, major local or national news coverage outside regularly scheduled newscast hours or a major national sporting event such as the Super Bowl or World Series, with other events such as the Indianapolis 500 and Olympic Games mentioned in individual EAS state plans.An RWT is not required during a calendar week in which an RMT is scheduled. No testing has to be done during a calendar week in which all parts of the EAS (header burst, attention signal, audio message, and end of message burst) have been legitimately activated.In July 2018, in response to the aftermath of the false missile alert in Hawaii earlier in the year (which was caused by operator error during an internal drill protocol), the FCC announced that it would take steps to promote public awareness and improve efficiency of the system, including requiring safeguards to prevent distribution of false alarms, the ability to authorize "live code" tests—which would simulate the process and response to an actual emergency, and authorizations to use the EAS tones in public service announcements that promote awareness of the system.

On February 3, 2011, the FCC announced plans and procedures for national EAS tests, which involve all television and radio stations connected to the EAS, as well as all cable and satellite services in the United States. They are not relayed on the NOAA Weather Radio (NOAA/NWS) network as it is an initiation-only network and does not receive messages from the PEP network.[22][23] The national test would transmit and relay an Emergency Action Notification on November 9, 2011 at 2:00 p.m. EST.The FCC found that only half of participants received the message via IPAWS, and some "failed to receive or retransmit alerts due to erroneous equipment configuration, equipment readiness and upkeep issues, and confusion regarding EAS rules and technical requirements", and that participation among low-power broadcasters was low. To reduce viewer confusion, the FCC stated that future national tests would be delivered under the new event code "National Periodic Test" ("NPT"), and list "United States" as its location.[26][27] A second national test, now classified as an NPT, occurred on September 28, 2016 as part of National Preparedness Month. A third national periodic test occurred on September 27, 2017.The fourth NPT occurred on October 3, 2018 (delayed from September 20, 2018 due to Hurricane Florence). It was preceded by the first mandatory wireless emergency alert test.The fifth NPT occurred on August 7, 2019, moved up from past years to prevent it from occurring during the heart of the Atlantic hurricane season. The test focused exclusively on distribution to broadcast outlets and television providers via the primary entry point network, in order to gauge the efficiency of alert distribution in the event that the internet cannot be used.The sixth NPT was postponed to 2021 amid the ongoing COVID-19 pandemic "out of consideration for the unusual circumstances and working conditions for those in the broadcast and cable industry." The sixth test is set to occur on August 11, 2021 with August 25 as a backup date. This test will involve the WEA system alongside television and radio.

The number of event types in the national system has grown to eighty. At first, all but three of the events (civil emergency message, immediate evacuation, and emergency action notification [national emergency]) were weather-related (such as a tornado warning). Since then, several classes of non-weather emergencies have been added, including, in most states, the AMBER Alert System for child abduction emergencies. In 2016, three additional weather alert codes were authorized for use in relation to hurricane events, including Extreme Wind Warning (EWW), Storm Surge Warning (SSW) and Storm Surge Watch (SSA).In 2004, the FCC issued a Notice of Proposed Rulemaking (NPR) seeking comment on whether EAS in its present form is the most effective mechanism for warning the American public of an emergency and, if not, on how EAS can be improved, such as mandatory text messages to cellphones, regardless of subscription. As noted above, rules implemented by the FCC on July 12, 2007 provisionally endorse incorporating CAP with the SAME protocol.In November 2020, Congress passed the Reliable Emergency Alert Distribution Improvement (READI) Act. First sponsored by Hawaii Senator Brian Schatz in response to the Hawaii false missile alert, it amends the Warning, Alert, and Response Network (WARN) Act to require distribution of wireless alerts issued by the administrator of FEMA, and commands the FCC to establish a means of reporting false alerts, encourage the establishment of State Emergency Communications Committees (SECC) that would meet annually to evaluate their EAS plans, require the repetition of alerts surrounding "emergencies of national significance", and open an inquiry into the feasibility of implementing the EAS on internet-related services.

The EAS is designed to be useful for the entire public, not just those with SAME-capable equipment. However, several consumer-level radios do exist, especially weather radio receivers, which are available to the public through both mail-order and retailers. Other specialty receivers for AM/FM/ACSSB (LM) are available only through mail-order, or in some places from federal, state, or local governments, especially where there is a potential hazard nearby such as a chemical factory. These radios come pre-tuned to a station in each area that has agreed to provide this service to local emergency management officials and agencies, often with a direct link back to the plant's safety system or control room for instant activation should an evacuation or other emergency arise.The ability to narrow messages down so that only the actual area in danger is alerted is extremely helpful in preventing false warnings, which was previously a major tune-out factor. Instead of sounding for all warnings within a station's area, SAME-decoder radios now sound only for the counties for which they are programmed. When the alarm sounds, anyone with the radio knows that the danger is nearby and protective action should be taken. For this reason, the goal of the National Weather Service is that each home should have both a smoke detector and a SAME weather radio.

The EAS can only be used to relay audio messages that preempt all programming; as the intent of an Emergency Action Notification is to serve as a "last-ditch effort to get a message out if the [p]resident cannot get to the media", it can easily be made redundant by the immediate and constant coverage that major weather events and other newsworthy situations—such as, most prominently, the September 11 attacks in 2001—receive from television broadcasters and news channels. Following the attacks, then-FCC chairman Michael K. Powell cited "the ubiquitous media environment" as justification for not using the EAS in their immediate aftermath. Glenn Collins of The New York Times acknowledged these limitations, noting that "no president has ever used the current [EAS] system or its technical predecessors in the last 50 years, despite the Soviet missile crisis, a presidential assassination, the Oklahoma City bombing, major earthquakes and three recent high-alert terrorist warnings", and that using it would have actually hindered the availability of live coverage from media outlets.Following the tornado outbreak of March 3, 2019, Birmingham, Alabama NWS meteorologist Kevin Laws told CNN that he, personally, wished that alerts could be updated in real-time in order to reflect the unpredictable nature of weather events, noting that the storm system's unexpected change in trajectory towards Lee County resulted in only a nine-minute warning (the resulting tornado would kill 23 people).