David Caruso as Det. John Kelly (Season 1 and four episodes of season 2)

Dennis Franz as Det./Sgt. Andy Sipowicz (Seasons 1-12

James McDaniel as Lt./Capt. Arthur Fancy (Seasons 1-8)

Sherry Stringfield as ADA Laura Michaels (Season 1)

Amy Brenneman as Officer Janice Licalsi (Season 1 and two episodes of season 2)

Nicholas Turturro as Ofc./Det./Sgt. James Martinez (Seasons 1-7)

Gordon Clapp as Det. Greg Medavoy (Seasons 1-12)

Sharon Lawrence as ADA Sylvia Costas (Seasons 2-5, recurring previously and after)

Gail O'Grady as Donna Abandando (Seasons 2-3, recurring previously)

The first pilot episode aired on September 21, 1993. Season1:John Kelly and Andy Sipowicz are detectives in the 15th squad. Sipowicz is the elder partner but is a drunk and a threat to the partnership lasting much longer. Kelly has a genuine affection for his partner but becomes increasingly exasperated by Sipowicz's behavior. In the pilot, Sipowicz is shot by a suspect that he had insulted in court. This leads to his decision to sober up and save his job. Whilst his partner is recuperating, Kelly is teamed up by the squad's Lieutenant, Arthur Fancy, with a young cop from Anti-Crime, James Martinez. Kelly's personal life is no less frenetic as he is going through a divorce from his wife, Laura, and is embarking on an affair with a uniformed cop, Janice Licalsi. To complicate matters further, Licalsi has been ordered to do a 'hit' on Kelly by mob boss Angelo Marino, otherwise Marino would turn in Licalsi's father, who is on his payroll. Instead, Licalsi murders Marino and the repercussions come back to haunt both her and Kelly. Sipowicz, meanwhile, sobers up and begins a relationship with A.D.A. Sylvia Costas whilst the other detective in the squad, Greg Medavoy, embarks on an affair of his own with the squad's new P.A.A., Donna Abandando.

Original cast from Season 2&3

Nicholas Turturro as Ofc./Det./Sgt. James Martinez (Seasons 1-7)

James McDaniel as Lt./Capt. Arthur Fancy (Seasons 1-8)

Rick Schroder as Det. Danny Sorenson (Seasons 6-8)

Gordon Clapp as Det. Greg Medavoy (Seasons 1-12)

Nicholas Turturro as Ofc./Det./Sgt. James Martinez (Seasons 1-7)

Bill Brochtrup as John Irvin (Seasons 7-12, recurring previously)

Later seasons: During the next two seasons, there are a few minor cast changes: Donna is replaced by several PAA's, most notably by Lourdes Benedicto, who plays Gina Colon, a character that eventually marries Martinez and is written out; and Andrea Thompson who plays Det. Jill Kirkendall and is partnered up with Russell. Sipowicz's battle with prostate cancer and the up-and-down Simone/Russell relationship, which included Russell's revelation that she had been sexually abused by her father. Also during this time, Franz would win four Emmy Awards, and both Delaney and Clapp would each win an Emmy for supporting roles.
Season 6 becomes a major turning point in the history of the series, as Smits decides not to renew his contract and leaves the show. His exit is explained as Simone becoming ill with an enlarged heart, shortly after marrying Russell in a civil ceremony, and his body's subsequent rejection of a heart transplant. Smits was replaced by Rick Schroder as Det. Danny Sorenson. Also during Season 6, two other critical incidents occur: the heroin overdose death of PAA Dolores Mayo (played by Lola Glaudini), and the shocking death of Costas, gunned down at the courthouse trial of the suspect accused in Mayo's death by her distraught father. Costas's final words of 'Take care of the baby' to Sipowicz leads to his total initial withdrawal from the squad. Yet, his keen perceptiveness allows him to gain a confession from the accused suspect, who tried to buy his way out of trouble. Furthermore, Sipowicz reaches a level of understanding with PAA John Irvin (portrayed by Bill Brochtrup), whose homosexuality was a foible for Sipowicz in their interactions to that point.
The next two seasons see the continuation of the Sipowicz/Sorenson relationship, along with more changes in the squad: departing during this time were Kirkendall, Martinez, Fancy as squad leader (through a promotion to write him out), and even Russell herself for a leave of absence to grieve the loss of Simone. Arriving to replace them would be Det. Baldwin Jones, played by Henry Simmons, Det. Connie McDowell played by Charlotte Ross, and Lt. Tony Rodriguez, played by Esai Morales. At the end of Season 8, Sorenson is approached by the owners of a strip club to work for them providing information and such. After reporting this to Lt. Rodriguez, Sorenson goes undercover, but then turns up missing after a stripper he was seeing turns up dead in his apartment (not by his doing as it turns out). The Sorenson character would be written out at the start of Season 9 at the request of Schroder, who wanted to spend more time with his family in Montana.
The fourth and final phase of the show would take place over the final four seasons. In addition to the 'Sorenson missing' storyline, Season 9 would also initially tie-in with the September 11 terrorist attacks. A suspect trades immunity for a robbery and shooting in exchange for information on a buried rug in Brooklyn that turns out to include Sorenson's dead body. Filling the void as partner for Sipowicz is newly promoted Det. John Clark, played by Mark-Paul Gosselaar. As with Simone and Sorenson, there is initial tension between Clark and Sipowicz, largely due to an old feud from years earlier involving Sipowicz and Clark's father, John Clark Sr. (played in guest spots by Joe Spano). Season 9 also sees the introduction of Det. Rita Ortiz played by Jacqueline Obradors.
The remaining four years would see a continuing focus on Sipowicz as the main character, as had been the case since Simone's death. Another unlikely romance would develop between Sipowicz and McDowell. This came about due to her ability to stand up to Sipowicz's gruffness, and her tender relationship with Theo (played by Austin Majors). They would eventually marry, and after adopting McDowell's sister's baby daughter (following the sister's murder by her husband, Connie's brother-in-law), they would have a child of their own as well. The McDowell character would eventually become an off-screen character only in the final two seasons, due to issues between Ross and show executives. Other departures and arrivals: Rodriguez would be written out following a dispute with an IAB captain who shot him in a drunken rage; replacing him initially as head of the squad was Sgt. Eddie Gibson, played by former actual NYPD officer John F. O'Donohue, who had previously served in the squad both on night watch and briefly on the 'day tour'; Gibson was then removed and replaced at the start of Season 12 by Lt. Thomas Bale, played by Currie Graham; arriving and then departing was ADA Valerie Haywood, played by Garcelle Beauvais-Nilon; and replacing McDowell was young Det. Laura Murphy, played by Bonnie Somerville. The final few episodes involve the impending retirement of Det. Medavoy, long the bumbling but well-meaning court jester in the show, and Sipowicz's attempts to take the Sergeants exam.