Towards the end of the fifth season, executive producer Dick Wolf decided not to renew Chris Noth's contract, citing that the interaction between Logan and the similarly jaded Briscoe (Jerry Orbach) lacked enough dramatic contrast. Viewers and critics were shocked at the dismissal of the most popular and only original remaining cast member at that time. Years later, Noth convinced Wolf to produce Exiled (1998) to wrap up the story of Mike Logan, which Noth felt had been prematurely extinguished on the show. Noth returned again to revive the character for two seasons of Law & Order: Criminal Intent (2001).

Jill Hennessy's twin sister, Jacqueline Hennessy, once played her sister's character during courtroom scenes filmed while Jill was unavailable, due to filming an appearance on Homicide: Life on the Street (1993).

Detective Mike Logan's brown leather coat was not supplied by the wardrobe department. Chris Noth bought that coat from a second-hand clothing store.

When the show began airing in re-runs on TNT, new digital technology was used to insert "product placements" (paid appearances of name-brand products) into the show. The easiest to spot is for Coca-Cola. Any time you see a Coke can sitting on a desk, it has been added digitally.

Many regulars made one-time guest appearances before becoming regulars, including: S. Epatha Merkerson, Jerry Orbach, Annie Parisse, Milena Govich, and Jeremy Sisto. Jesse L. Martin was hired for a small role, but decided not to take it.

Prior to becoming an actor, Dennis Farina (Detective Joe Fontana) was a Detective in the Chicago Police Department for eighteen years. He got into acting through his former partner and police commander, Chuck Adamson, who had retired and became a police consultant on movies and television shows.

Jerry Orbach left the show after twelve years to star in the spin-off Law & Order: Trial by Jury (2005). He died after filming only two episodes.

The last of the original cast members was Chris Noth. Although Steven Hill was often touted as an original cast member, he joined after the first episode.

During the 2008 Presidential election season, TNT stopped re-running episodes of Law & Order that featured Fred Dalton Thompson as District Attorney Arthur Branch. Thompson was running for the Republican nomination for President, and due to federal election law, TNT would have been required to give other candidates equal air time. Thompson eventually dropped out of the race.

The distinctive "thunk-thunk" sound effect used in between scenes was created by combining close to a dozen sounds, including that of a group of monks stamping on a floor. The sound is intended to be reminiscent of both a judge's gavel, and a jail cell door slamming.

In 2001, creator Dick Wolf announced plans for a special Law & Order mini-series featuring the casts of the original, Law & Order: Special Victims Unit (1999), and Law & Order: Criminal Intent (2001). The mini-series was to have been broadcast in the spring of 2002, and deal with a terrorist attack on New York. After the World Trade Center attacks of September 11, 2001, the mini-series was cancelled.

Steven Zirnkilton, who narrates the opening credit sequences, only appeared on-screen once. He played a detective in the pilot episode Law & Order: Everybody's Favorite Bagman (1990). He had one line of dialogue: "Look at that. Do you believe these guys?"

Paul Robinette (Richard Brooks) and Jamie Ross (Carey Lowell) played Assistant District Attorneys and, after leaving the show, returned and played defense attorneys.

Steven Hill and Sam Waterston were both 68 years old when they started playing the District Attorney. Hill turned 68 in 1990, Waterston turned 68 in 2008.

It is said that District Attorney Adam Schiff is loosely based on the real Manhattan District Attorney Robert Morgenthau, and it's been reported that Morgenthau himself enjoys the character of Schiff.

Jerry Orbach auditioned for the role of Sergeant Max Greevey, but was beat out by George Dzundza. Orbach later joined the show as Detective Lennie Briscoe.

With the airing of its twentieth season, beginning September 25, 2009, this show tied the record held by Gunsmoke (1955) for the longest running prime-time drama series on U.S. television. However, it was announced in May 2010 that the series was not renewed for the 2010-2011 season. This longevity record was tied by its spin-off Law & Order: Special Victims Unit (1999) in September 2018.

At the time of the show's creation, one-hour dramas were going through a slump, with sitcoms being more popular, and much more likely to get strong syndication deals for re-runs. Dick Wolf thought that it might be easier to sell the show in thirty-minute segments, and came up with the concept of the first half of the show being the police investigation and the second half the legal procedure. Dramas started rebounding in popularity shortly after Law & Order debuted, so this never ended up becoming an issue with re-run deals.

Michael Moriarty resigned at the end of the fourth season after a long, vocal battle of words with Attorney General Janet Reno, who was making efforts to censor television violence. He felt that NBC was trying to silence him when two talk show appearances on the network were pulled at the last moment and his role was reduced considerably in the fourth season episode "Mayhem." Dick Wolf claims this was entirely coincidental. Moriarty claims he was forced into a situation where he had to resign. His character, EADA Ben Stone, also resigned on the show. After quitting the series, Moriarty moved to Canada, where he considered forming a political party.

After her departure from the series, Jill Hennessy wrote and directed The Acting Class (2000) using Law & Order's sets. Many Law & Order cast members, such as Jerry Orbach and Angie Harmon volunteered to appear in her film, which was a mock-documentary.

The series was originally to air on FOX. But Fox CEO Barry Diller later decided against it. The series was then pitched to CBS, and the pilot episode "Everybody's Favorite Bagman" was produced in 1988. But CBS did not pick up the series. NBC picked it up in 1989, and the series began in 1990.

Dann Florek and Richard Brooks were both let go after the third season, because producers felt the show needed female characters. They were replaced by S. Epatha Merkerson and Jill Hennessy respectively. Dann Florek was brought back for Law & Order: Special Victims Unit (1999), and Richard Brooks returned in a few episodes of Law & Order to play the same character but as a defense attorney.

S. Epatha Merkerson wears her hair in interlocks. Although she has played police officers in other shows with her own hairstyle, notably Captain Margaret Claghorn in the futuristic crime series Mann & Machine (1992), she felt it would be unrealistic for an ambitious New York City Police Lieutenant to adopt such a hairstyle, so she wears a wig as Lieutenant Van Buren.

The show was based on and inspired by a BBC 4 part drama about the British legal system broadcast in 1978 and seen by Dick Wolf. The drama looked at an event ( a corrupt Scotland Yard detective frames a known criminal for an armed robbery) from four different perspectives: the policeman's, the criminal's, the prosecution and defence lawyers and finally life in prison for the unfairly convicted villain. Wolf liked the innovative idea of following a crime from both the police investigation and then the lawyers perspective through to the trial and verdict.

The show was known for underplaying the background stories of its characters. During the 1995-96 season, hints were dropped that the characters of Jack McCoy and Claire Kincaid were lovers (a fact confirmed in a later episode). Many fans enjoy spotting where and when these subtle hints occur in each episode. However, when Elisabeth Röhm's character Assistant District Attorney Serena Southerlyn came out as gay in her very last line on the show (after she is fired, Southerlyn asks, "Is this because I'm a lesbian?") after absolutely no other indication of her character's sexual orientation had been given during her four years on the show, the writers came in for widespread derision from television critics (including Slate's Dana Stevens, USA Today's Robert Bianco, and Tim Goodman of the San Francisco Chronicle) and on internet message boards for using the revelation of her orientation for its shock value instead of allowing it to be any part of her character.

Jesse L. Martin was absent from the last four episodes of the fifteenth season. He left the show temporarily to work on Rent (2005). His character, Detective Green, was shot in the line of duty. For those four episodes, Martin was replaced by Michael Imperioli as Detective Falco.

S. Epatha Merkerson announced that she was leaving the series after the twentieth season, but the show was later cancelled after that season.

The distinctive typeface used for all titles, credits, and on-screen "scene change" cards, for this show, and all of its spin-offs, is named "Friz Quadrata." The typeface used for "Starring" and "Created By" in the opening credits, the lone exception, is Eurostile.

All three of the series' longest-serving cast members lasted far longer than those they replaced: Jerry Orbach (Detective Lennie Briscoe) (1992-2004) lasted twelve years and replaced Paul Sorvino (Sergeant Phil Ceretta) who lasted only a year and a half (1991-1992); S. Epatha Merkerson (Lieutenant Anita Van Buren) (1993-2010), was with the series for seventeen seasons, replaced Dann Florek (Captain Don Cragen), who lasted three years (1990-1993); Sam Waterston (Executive District Attorney/District Attorney Jack McCoy) (1994-2010), who was with the series for sixteen seasons, replaced Michael Moriarty (Executive Assistant District Attorney Ben Stone), who lasted four years (1990-1994) and all three of the series' longest-serving cast members that were not there when the series ended lasted far longer than the ones that replaced them. Jerry Orbach (Detective Lennie Briscoe) (1992-2004) lasted twelve years and was replaced by Dennis Farina (Detective Joe Fontana) (2004-2006) who lasted two years. Steven Hill (District Attorney Adam Shiff) (1990-2000) lasted ten years and was replaced by Dianne Wiest (District Attorney Nora Lewin) (2000-2002) who lasted two years Jesse L. Martin (Detective Ed Green) (1999-2008) lasted nine years and was replaced by Anthony Anderson (Detective Kevin Bernard) (2008-2010) who lasted two years.

George Dzundza left the series after the first season, because he tired of the commute to New York City from his Los Angeles house. He was replaced by Paul Sorvino. Sorvino left in the third season, because he disliked show's work schedule, and, because he was an opera singer, he wanted to preserve his vocal cords.

Law & Order used several actors to play multiple characters. Edward D. Murphy portrayed twelve different characters in twelve episodes during the first ten seasons. Lee Shepherd portrayed seven different characters in nine episodes in nine seasons. Isiah Whitlock Jr. plays a man that confesses to the murder of several children in one episode. In a later episode, he plays an NYPD Captain. In all, he appears six times in five roles. Dennis Boutsikaris appeared 7 times (1990-2004). The actors who do this are called repeat offenders.

Dr. Emil Skoda (J.K. Simmons) was named after the founder of the Czech engineering and manufacturing firm, Skoda.

There are a total of five spin-offs of the original Law & Order (1990). They are: Law & Order: Special Victims Unit (1999), Law & Order: Criminal Intent (2001), Law & Order: Trial by Jury (2005), Law & Order: UK (2009), Law & Order: LA (2010) and Law & Order True Crime (2017)

Season eight (1997-98) marks the first season of the series in which there were no main character changes. From season two on, there was at least one change in the main characters, either at the beginning or during the season.

Michael Madsen was a finalist for the role of Detective Mike Logan.

Many of the actors on Law & Order were also regular cast members on HBO series The Wire (2002). In fact, at least twenty actors that made a special appearance on Law & Order went on to major story lines in "The Wire". For instance, Peter Gerety played an attorney on Law & Order, and on "The Wire", he played a judge who authorized the wiretap which was the initial premise of the show.

The telephone records, LUDs, is Local Usage Details.

Danny Trejo and John Leguizamo were both considered for the role of Detective Reynaldo "Rey" Curtis.

Steve Burns of Blue's Clues (1996) made his television debut in the season five episode "Cruel and Unusual."

Law & Order episodes are often advertised as being "ripped from the headlines." Many people mistake this to mean that they are based on real events. In reality, the slogan is referring to the show's practice of coming up with stories that are partially inspired by recent headlines. However, with almost no exceptions, only a fairly small portion of the episode will resemble the real incident or incidents that it is inspired by. There might be a few scenes that resemble a well-known headline while the majority of the episode goes in a different direction, or there could be one character that is based on a famous individual, but the circumstances the person encounters are largely made up.

Martin Sheen was considered for the role of Sergeant Max Greevey.

Eriq La Salle was a finalist for the role of Assistant District Attorney Paul Robinette.

Steven Hill plays the Manhattan District Attorney, a role he played in Legal Eagles (1986), in which Robert Redford plays the Assistant District Attorney, and is fired by Hill.

Vanessa Williams auditioned for the role of Assistant District Attorney Abbie Carmichael. Angie Harmon was cast because Dick Wolf liked her Southern accent.

The show's format was in part inspired by Arrest and Trial (1963).

The series was filmed on the same Manhattan studio where The Blacklist (2013) was filmed.

Lorraine Toussaint played defense attorney Shambala Green opposite prosecutor Ben Stone (Michael Moriarty) in six episodes of Law & Order (1990) from 1990 to 1994. In a 1998 interview, Toussaint said that she and Moriarty got along so well, and had such potent onscreen chemistry together, that Moriarty asked producer Dick Wolf to develop a love interest story arc for the two characters. This idea was rejected. Moriarty left "Law & Order" after its fourth season, and Toussaint would play Shambala Green only twice more: in a 2003 episode of "L&O", and in a 2016 episode of the Dick Wolf-produced series Chicago P.D. (2014).

Chris Noth (Mike Logan) & Leslie Hendrix (Dr. Elizabeth Rodgers) also worked together on The Good Wife (2009) & Law & Order: Criminal Intent (2001).

Chris Noth (Mike Logan) & Jill Hennessy (Claire Kincaid) also worked together on three episodes of The Good Wife (2009), as Peter Florrick & Rayna Hecht respectively.

Jerry Orbach was a talented singer who performed on Broadway; however, his character Detective Lennie Briscoe expresses disdain for opera several times. Operas of which he speaks scornfully include DON GIOVANNI (Mozart), TRISTAN AND ISOLDE (Wagner), LA TRAVIATA (Verdi), and LA BOHEME (Puccini). Also, in one episode (Law & Order: Payback (2004)), when a suspect he is arresting complains that he has tickets for the Metropolitan Opera that evening, Briscoe replies: "Well, you're in luck. It's 'opera-week' at Rikers. They're doing an all-male version of CARMEN." Arthur Branch, on the other hand, is enough of an opera buff that he once sang the famous aria "Vesti la giubba ( "Ridi, Pagliacco" ) from Leoncavallo's opera Pagliacci (The Clowns) outside a girl's window, wearing full clown costume. (See Law & Order: Chosen (2003)). Since the aria is written for tenor voice, and Fred Dalton Thompson's voice is a low baritone, it must be assumed either that he transposed the music down, or, that his voice changed a great deal between his college days and the time when he became a prosecutor, or, that he was very very drunk when he sang.)

The series was originally set to film in Los Angeles, but Dick Wolf fought NBC to film it in New York City, and won. In May 2010, it was announced that a new Los Angeles-based spin-off, tentatively titled Law & Order: LA (2010), would start in the 2010 fall line-up.

Stephen Berger has portrayed three different judges throughout the series: Episode 6.19 Law & Order: Slave (1996) - Judge Robin Ingles Episode 11.13 Law & Order: Phobia (2001) - Judge David Weintraub Episode 15.19 Law & Order: Sects (2005) - Judge Thomas Huce

Chris Noth (Mike Logan) & Richard Brooks (Paul Robinette) also worked together on The Good Wife (2009), as Peter Florrick & Ace Barnstone respectively.

Stephen Tobolowsky auditioned for the role of Sergeant Max Greevey.

James Naughton was a finalist for the role of Executive Assistant District Attorney Ben Stone.

Jill Hennessy (Claire Kincaid) and Michael Rooker (Jamie Yost) were on Crossing Jordan (2001).

Chris Noth (Mike Logan) & Josh Pais (Assistant M.E. Borak) also worked together on episode 5.12, The Good Wife: We, the Juries (2014), of The Good Wife (2009) and episode 7.4, Law & Order: Criminal Intent: Lonelyville (2007), of Law & Order: Criminal Intent (2001).

Detective Briscoe's birthday is January 1, 1940.

After Michael Moriarty's resignation at the end of season four, he was replaced by Sam Waterston as Jack McCoy at the beginning of season five. Many years earlier, the two actors starred together in the TV movie The Glass Menagerie (1973).

Detective Green's shield number is 3472.