The second episode of series one gained notoriety for the violent torture and killing of character Helen Flynn with a deep fryer, which drew the largest number of complaints to the Broadcasting Standards Commission in 2002, despite airing after the 9pm watershed.

The fourth series opener, which was already filmed and involved a terrorist bombing storyline in central London, was due to air just two months after the real 7 July London bombings, prompting BBC drama chiefs to agonise over whether to pull the episodes before eventually airing them unedited with a viewer warning.

Actress Olga Sosnovska's character Fiona Carter was killed off in series four because Sosnovska was pregnant during filming and chose to leave the show.

Because of security risks around filming the real MI5 headquarters, London's Freemasons' Hall was used for scenes of characters entering or leaving Thames House and for some interior locations; the same building was later used as Thames House in Torchwood: Children of Earth.

The series was shot on Super 16 film rather than the more commonly used digital formats of the era.

Episodes originally aired with no opening credits on BBC One, a deliberate choice to preserve an atmosphere of anonymity befitting real-life spies; series six was the first to include end credits.

In the final episode of series two, music from the film score Spy Game by Harry Gregson-Williams (tracks 'Beirut, a War Zone' and 'Operation Dinner Out') was used instead of the show's regular score, and broadcast versions of episodes sometimes featured different music than the commercial DVD releases.

A spin-off, Spooks: Code 9, was announced in 2006 following the success of Torchwood and began filming in Bradford in 2008, but it was poorly received by critics who called it 'daft and unconvincing' and said it 'damages the Spooks brand'.