Season 1
Season 1 premiered March 12, 2002. It gives an introduction to The Strike Team and the other characters of the Barn. Important plotlines are the aftermath of Vic's murder of Terry Crowley and Captain Aceveda's scheming to bring Vic and the Strike Team down; Dutch and Claudette's attempts at tracking down a serial killer; Julien's training under Danny and his struggle with his homosexuality; Vic's use of Rondell Robinson to control the local drug trade and the fallout; as well as the corrupt actions of Ben Gilroy.

Season 2
Season 2 premiered January 7, 2003. The season mostly revolves around a brutal new drug lord, Armadillo-a sadistic child rapist, who likes to set his rivals on fire using a tire necklace and gasoline-who begins to take over the drug trade in Farmington. Meanwhile, Officer Sofer is involved in a shooting of a Muslim man and has to deal with the fallout. This season is also heavily concerned with the Strike Team's plan to rip off the money train of the Armenian Mob, which ends up going down in the season finale.

Season 3
Season 3 premiered March 9, 2004. The season mainly revolves around the aftermath of the Money Train Heist and its effects on the Strike Team, as the Armenian mob and also David Aceveda begins to suspect the Strike Team. In order to save the team, Lem (Curtis "Lemonhead" Lemansky) burns a majority of the money, ultimately leading to a confrontation which causes the Strike Team to split up in the season finale. The Armenian mob sends Margos Dezerian to wipe out the Strike Team. Dezerian leaves a trail of murders, resulting in his own execution at the hands of Mackey. Claudette had been promised a promotion to captain and throughout the season was in a supervising role, while Aceveda prepared to move onto city council. Near the end of the season an assistant district attorney was shot, and Wyms and Dutch discovered she had been a heavy drug user for the past 3 years. Wyms explored further and became very unpopular with the D.A. and around the Barn because she was (against orders) reopening the assistant DA's closed cases. This resulted in her being denied her promotion to captain of the Farmington District.

Season 4
Season 4 premiered March 15, 2005, with the addition of Glenn Close taking over the role as Farmington's new captain, Monica Rawling. The season dealt with the fallout from the Strike Team disbandment. Shane Vendrell, with new partner Army, enters into a dangerous situation with major drug lord Antwon Mitchell (Anthony Anderson), and seemingly accepts an order to kill Vic Mackey. The police were outraged after two officers were kidnapped and subsequently found murdered. In the end, the Strike Team is re-formed and manages to successfully put Antwon in prison. The season also deals with the controversial asset forfeiture policies of the new captain; Julien's opposition to these policies; and David Aceveda (now a City Councilman) dealing with the psychological aftermath of his sexual assault incident from the previous season. The season concludes with Captain Rawling losing her job over a dispute with the DEA. This plot twist reflected a real-life need for Glenn Close to return to New York.
One of the season's secondary plots involves Claudette and Dutch's marginalization as detectives because of Claudette's refusal to apologize to the DA for reopening the cases of a public defender who was discovered to be a functioning drug addict. Claudette's moral stand resulted in many of the prosecutor's cases being overturned. This cost Claudette her shot at becoming Farmington Captain. Dutch eventually resolved the situation by making a back-room deal with the DA to "keep Claudette in line" and do favors for the office in return for breaking back into action.

Season 5
Season 5 premiered January 10, 2006. The season revolved around Internal Affairs Department Lt. Jon Kavanaugh's (played by Forest Whitaker) investigation into the Strike Team, representing one of the greatest threats the team has ever faced. As a result of Kavanaugh turning one of Vic's informants, IAD became aware of Lem stealing heroin which he never turned in. Having found the heroin, IAD is capable of arresting Lem, but Kavanaugh wants him to incriminate the whole team and has him wear a wire, which he warns the team of and they use it to embarrass IAD. Kavanaugh puts on any form of pressure he can, finds out about Vic's share of the Money Train money, and ultimately arrests Lem having made a deal with Antwon Mitchell to put Lem there if convicted. Vic stands with him and gets bail, while Shane is worried he'll be turned. Claudette finally gets her shot as the captain of the Barn which she reluctantly accepts. The season concluded with Shane Vendrell murdering his friend and fellow team member Lem with a hand grenade.

"Wins and Losses"
The producers of The Shield produced a 15-minute "promosode", which premiered on Google on February 15, 2007 to bridge the gap between Seasons 5 and 6. The episode focuses on the aftermath of Lem's death, including his funeral and flashbacks as co-workers reflect upon Det. Lemansky's life. The episode was said to have cost between $500,000 and $1 million to produce and was on Bud.TV for a 4 week period and later released to AOL and other media outlets.[5] The "promosode" is also one of the special features included on the Season 5 DVD set.

Season 6
Season 6 premiered on FX on April 3, 2007. Continuing directly after season 5, Vic and the Strike Team are distraught over Lem's death. Shane has been overcome by guilt and becomes reckless and suicidal. Kavanaugh refuses to let the case die and resorts to planting evidence and coercing witnesses to lie about the Strike Team and specifically, Vic. Dutch and Claudette begin to suspect his integrity and Kavanaugh finally confesses to his actions and finds himself under arrest. Vic learns from Claudette that the Chief plans to force him into early retirement - and vows to wreak bloody vengeance on Lem's killer before losing his badge. Claudette learns that the Barn could be shut down if no improvements are made by the time quarterly crime statistics are released. The season concludes with the breakdown of Vic and Shane's friendship and Shane getting in over his head with the Armenians.
Season 6 was originally intended to be aired as the second half of Season 5 (in the same way that HBO split up the last season of The Sopranos); FX decided to refer to these ten episodes as "Season 6" instead.

Season 7
Season 7 premiered September 2, 2008 consisted of 13 one-hour episodes and a ninety-minute finale, first aired in the US on FX on November 25, 2008. Vic's ex-wife Corinne has learned of his many crimes and agrees to work with Dutch and Claudette to try and send him to prison. Ronnie is also implicated in the process. Shane, Mara and Jackson go on the run and Dutch has problems of his own while dealing with a teenage serial killer. As part of an immunity deal with ICE, Vic admits to every crime the Strike Team has committed and implicates Shane and Ronnie in enough to send them to jail for life. After learning that there is no way to escape prison, Shane commits suicide after poisoning his pregnant wife and five-year old son. Claudette reveals the terminal status of her illness to Dutch, who promises to stand by her as a friend. Ronnie is arrested in front of Vic at the barn. Desperate to escape Vic, Corinne and the children disappear into the witness protection program. Aceveda stands on the verge of being elected mayor. Meanwhile, Vic is left trapped in a desk job at ICE, presumably loathed by his co-workers and ostracized by his fellow cops, who want nothing to do with him now that his many crimes have been exposed.