The Knock is a British television crime drama, created by Anita Bronson and broadcast on ITV, which portrayed the activities of customs officers from the London City & South Collection Investigation Unit of HM Customs and Excise. The series derived its name from the distinctive "Knock knock knock" command used over the radio to synchronise a raid.

Five series were broadcast from 10 April 1994, until 11 November 2000. 37 episodes were made. The series had a rotating cast, with only a small number of cast members appearing throughout the series' run. The only three cast members to appear in every series were Caroline Lee-Johnson, Trevor Byfield and Steve Toussaint. The series also adopted a number of different formats: while the early series interspersed various storylines and had running plots across the series; later series adopted a multi-part format resulting in two or three cases per series; while the final series adopted a stand-alone week by-week format.

The series was axed in 2001 following poor viewing figures for the final series. This was blamed on the loss of several main cast members, a change in the format and the overall look of the series which changed dramatically following an overhaul by ITV executives in 1999.