This chapter studies the use of the controversial surveillance method called ‘Special Facilities’ (SF), the use of which historically MI5 to this day seems keen to conceal. SF was the code name for telephonic electronic eavesdropping, and sometimes for the use of microphones by MI5 for covert purposes in any circumstances. SF in this former sense was often, but not always, used in conjunction with a telephone interception, which was almost invariably set up at the telephone exchange, or at least outside the target’s home or office. The legal questions raised by the use of SF were never properly addressed until 1989. At the heart of it, however, is the question of whether the placing and use of covert listening devices was lawful.