In the Star Trek fictional universe, Section 31 is the name of an unofficial intelligence and defense organization. It is presented as a special security operation, manned by United Federation of Planets citizens, that is not subject to the normal constraints of Starfleet ethical protocols. The organization appears, or is mentioned, across eight episodes of Star Trek: Enterprise and Star Trek: Deep Space Nine.
Section 31 is comparable to other secret police organizations in the Star Trek universe, such as the Romulan Tal Shiar and the Cardassian Obsidian Order. Unlike those organizations, Section 31 does not exist to enforce government policy or punish traitors, but rather to defend against external threats. Also, unlike the Tal Shiar and Obsidian Order, both of whom are greatly feared within their populaces, most people in the Star Trek universe have never heard of Section 31 and will never encounter it.
Little of Section 31's history has been revealed on-screen. Since the concept of Section 31 was created for the Deep Space Nine episode Inquisition, most references to the organization appear in episodes of Deep Space Nine, although Section 31 also appears in Star Trek: Enterprise. Several works of Star Trek spin-off fiction expands on Section 31's operations; Pocket Books published a four-part series profiling connections between Section 31's operations and the missions of James T. Kirk, Jean-Luc Picard, and the crews of Deep Space Nine and the USS Voyager. These novels explicitly link Section 31 to Fleet Admiral Cartwright's actions in Star Trek VI: The Undiscovered Country and Vice Admiral Matthew Dougherty's actions in Star Trek: Insurrection. Section 31 is also heavily featured in the Star Trek: Enterprise novel The Good That Men Do, in which Trip Tucker joins the organization after his supposed "death".
Section 31 agents include the following characters: