Sir Andrew Leggatt, the first Chief Surveillance Commissioner, described the acronym CHIS (Covert Human Intelligence Source) as 'unlovely' (Office of Surveillance Commissioners, 2002, paragraph 4.8). Syntactical particularity aside, as a generic label 'CHIS' is certainly of use to Treasury Counsel in drafting legislation that prescribes authority regimes, but such linguistic utility obscures the key differences between different types of CHIS, which are important when considering regimes of regulation. For the purposes of this chapter, a more functional nomenclature is preferred - informer and infiltrator - to highlight the different implications present in these roles. (This chapter avoids use of 'informant' as this term also applies to members of the general public who contact the police to report a crime or accident, for instance.) Undercover infiltration being a particular specialism, reserved for the investigation of more serious crime, the primary focus of this chapter, as in most of the available literature, is on informers.