ER is an American medical drama television series created by Michael Crichton that aired on NBC from September 19, 1994, to April 2, 2009, with a total of 331 episodes spanning 15 seasons. It was produced by Constant c Productions and Amblin Television, in association with Warner Bros. Television. ER follows the inner life of the emergency room (ER) of Cook County General Hospital, a fictionalized version of the real Cook County Hospital, in Chicago, and the various critical professional, ethical, and personal issues faced by the department's physicians, nurses, and staff.

The show is the second-longest-running primetime medical drama in American television history behind Grey's Anatomy. The highest-awarded medical drama, ER won 128 industry awards from 442 nominations, including the Peabody Award, TCA Award for Program of the Year, and Primetime Emmy Award for Outstanding Drama Series. As of 2014, ER had grossed over $3 billion in television revenue. It is considered one of the best medical dramas of all time, pioneering the field of medical fiction and setting a model for other contemporary medical dramas to follow.

In 1974, author Michael Crichton wrote a screenplay then titled "EW" (for Emergency Ward) based on his own experiences as a medical student in a busy hospital emergency room. Producers were not interested in the screenplay, and Crichton turned to other topics. In 1990, he published the novel Jurassic Park, and in 1993 began a collaboration with director Steven Spielberg on the film adaptation of the book.

After its release, Crichton and Spielberg then turned to what was now known as ER, but Spielberg decided to film the story as a two-hour pilot for a television series rather than as a feature film after considering the potential for various stories to be told in the setting. He passed the script on to a team at his production company, Amblin Entertainment. Anthony Thomopoulos, then head of Amblin's television division, got in touch with then CEO of Warner Bros. Television, Les Moonves, about the idea for the series and to send the script. Spielberg's Amblin Television provided John Wells as the show's executive producer.

Warner Bros. Television pitched ER to NBC, alongside Crichton, Spielberg, and Wells. Warren Littlefield, head of NBC Entertainment at the time, liked the project, but there was much debate and controversy among other executives at NBC, who were dubious about the nature of the series. NBC offered a chance to make a two-hour made-for-TV movie from the script, which was rejected. They then tried to get the show greenlit at rival networks before returning to NBC, who this time around ordered a pilot.

The script used to shoot the pilot was virtually unchanged from what Crichton had written in 1974. The only substantive changes made by the producers in 1994 were that a male character was changed to a female character (Susan Lewis) and the Peter Benton character's race was changed to African-American. The running time was shortened by about 20 minutes in order for the pilot to air in a two-hour block on network TV. Because of a lack of time and money necessary to build a set, the pilot episode of ER was filmed in the former Linda Vista Hospital in Los Angeles, an old facility that had ceased operating in 1990. A set modeled after Los Angeles County General Hospital's emergency room was built soon afterward at the Warner Bros. Studios in Burbank, California, although the show makes extensive use of location shoots in Chicago, most notably the city's famous "L" train platforms.

Littlefield was impressed by the series: "We were intrigued, but we were admittedly a bit spooked in attempting to go back into that territory a few years after St. Elsewhere. With Spielberg attached behind the scenes, NBC ordered six episodes. "ER premiered opposite a Monday Night Football game on ABC and did surprisingly well. Then we moved it to Thursday and it just took off," commented Littlefield. ER's success surprised the networks and critics alike, as David E. Kelley's new medical drama Chicago Hope was expected to crush the new series, airing directly opposite ER in the Thursday 10:00 pm time slot over on CBS.

Crichton was credited as an executive producer until his death in November 2008, and also posthumously for the final season. Wells, the series' other initial executive producer, served as showrunner for the first four seasons. He was the show's most prolific writer and became a regular director in later years. Lydia Woodward was a part of the first season production team and became an executive producer for the third season. She took over as showrunner for the fifth season while Wells focused on the development of other series, including Trinity, Third Watch, and The West Wing. John Wells continued to serve as the primary Executive Producer for the remainder of the series. Lydia Woodward left her executive producer position at the end of the sixth season but continued to write episodes throughout the series run.