The X-Files unit

The FBI's unsolved and unexplained cases, worked out of a secluded basement office where Scully first finds Mulder in the Pilot. The unit is shut down at the end of the first season and Skinner reopens it in Ascension, with the words that reopening is what the conspirators fear most.

Spooky

Mulder's nickname from the FBI Academy, as Scully tells the section chief in the Pilot. An Oxford educated psychologist and a respected profiler, he earned it by chasing cases nobody else would touch.

Mythology and monster of the week

Fans and critics split the series into two kinds of hour. Mythology episodes advance the long conspiracy story about aliens and the men hiding them; monster of the week episodes are standalone cases, from Tooms to the vampires of Bad Blood, and many of the show's best loved hours belong to the second group.

Walter Skinner

The FBI Assistant Director, played by Mitch Pileggi, who supervises the X-Files from his first appearance in Tooms. A Marine and Vietnam veteran, he begins as a skeptical boss answering to unseen powers and ends as the agents' most reliable ally, the man who shoots Krycek in Existence and defends Mulder at his trial.

Deep Throat

Mulder's first informant, played by Jerry Hardin and named for the Watergate source. He appears in the second episode and is gunned down in the first season finale, The Erlenmeyer Flask, after helping Scully secure an alien fetus. His last words are "Trust no one."

X

The informant who follows Deep Throat, played by Steven Williams. He first calls Mulder in The Host and appears on screen in Sleepless; Mulder summons him with a masking tape X on his apartment window. He is shot at Mulder's door in Herrenvolk and writes SRSG in his own blood, the clue that leads to Marita Covarrubias.

Marita Covarrubias

The third informant, played by Laurie Holden, introduced at the end of Herrenvolk as assistant to the Special Representative of the United Nations Secretary General. She first helps Mulder with the Tunguska investigation, then plays both sides.

The Lone Gunmen

John Fitzgerald Byers, Melvin Frohike and Richard Langly, the three conspiracy publishers behind the newsletter The Lone Gunman, first seen in E.B.E. They got a 13 episode spin-off in 2001 and died together in the ninth season episode Jump the Shark.

Alex Krycek

Nicholas Lea's double agent, introduced in Sleepless as Mulder's temporary partner and quickly revealed as the Smoking Man's plant. He loses his left arm in the Tunguska storyline, changes sides more than once, and is shot dead by Skinner in the eighth season finale, Existence.

The Cigarette Smoking Man

The series' central villain, played by William B. Davis, silent in the Pilot and present until My Struggle IV. Known to fans as Cancer Man, he goes by hundreds of aliases, C.G.B. Spender among them, and is the father of Agent Jeffrey Spender. In The Truth he is apparently killed by a helicopter rocket, only to return burned and disfigured in the 2016 revival.

Morley

The fictional cigarette brand the Smoking Man chain smokes, with a pack that resembles Marlboro's. Morleys are a long standing Hollywood prop brand sold by the Earl Hays Press, and the red and white packet became one of the show's most recognizable objects.

The Syndicate

The shadow group of American and international power brokers who hid the alien presence after Roswell in 1947 and met in a nondescript building on West 46th Street in New York. On October 13, 1973 they formally allied with the colonists, taking an alien fetus for research and giving up family members in return. Faceless alien rebels incinerated the senior members in One Son.

The Well-Manicured Man

The Syndicate's British voice of reason, played by John Neville from The Blessing Way onward. He prefers manipulation to violence and occasionally feeds Mulder clues. In Fight the Future he hands Mulder a vaccine and Scully's location, then dies by car bomb before the betrayal can be discovered.

Samantha Mulder

Mulder's sister, taken from the family home on Martha's Vineyard on the night of November 27, 1973, when she was eight and he was twelve. Her father surrendered her as part of the Syndicate's bargain. Closure, in season seven, reveals she was raised alongside Jeffrey Spender, ran away at fourteen and was taken by walk-ins before the men who held her could reclaim her.

The Project and the hybrids

The Syndicate's long effort, begun with German scientists brought over after the war, to breed an alien human hybrid that would serve the colonists after they arrived. Its products include the green blooded clones of Colony and End Game, the Gregors and the adult Samanthas, the Kurt Crawfords of Memento Mori, and Cassandra Spender, the first successful hybrid made by other means.

The colonists

The alien species whose ship crashed in New Mexico in 1947 and whose plan to retake the planet drives the mythology. The Syndicate served them while quietly trying to survive them, and in Fight the Future the group learns that an infected host does not become a slave but an incubator.

The black oil (Purity)

The colonists' own life form, an alien virus living in petroleum deposits that enters a host through the mouth, eyes or nose and takes control. First seen in Piper Maru, it is called Purity by the Syndicate and the black cancer by the Russians; the Syndicate's vaccine effort carried the code name Purity Control.

Colonization

The planned alien takeover of Earth. The Truth, the 2002 series finale, puts the date at December 22, 2012, the day the Smoking Man says the Mayan calendar stopped. The 2016 revival notes that the date passed without an invasion.

The alien bounty hunter

The shapeshifting enforcer, played by Brian Thompson, who first appears in Colony hunting the clones. His blood is green and corrosive, and he can be killed only by a stiletto-like alien weapon driven into the base of the neck.

The faceless rebels

Aliens of the bounty hunters' species who have sealed their own eyes and mouths against black oil infection and fight colonization. Introduced in Patient X, they burn abductees alive and in One Son incinerate the Syndicate.

The bees

The conspiracy's delivery system. Mulder finds them tended by drone clones in the Alberta fields of Herrenvolk; in Zero Sum a swarm carrying smallpox attacks a South Carolina schoolyard; and in Fight the Future a single bee infected with the black oil stings Scully.

Super soldiers

The eighth and ninth season threat: human replacements created by the colonists who bleed red, cannot shapeshift and are nearly impossible to kill. Billy Miles and Knowle Rohrer are the best known, and their one weakness is magnetite, which tears their bodies apart.

Scully's abduction

Duane Barry takes Scully from her apartment in Ascension and she returns, comatose, in One Breath, with no memory of the missing months. The implant later found in her neck and the cancer that follows tie her to a wider group of abducted women.

The MUFON women

A group of women from the Mutual UFO Network whom Scully meets in Nisei. They recognize her, carry implants like hers and are dying of cancer. By Memento Mori nearly all of them, including Betsy Hagopian, are dead; Penny Northern, the last, dies during Scully's own treatment.

William

Scully's son, born in the eighth season finale Existence in an abandoned Georgia town, delivered by Monica Reyes while super soldiers surround the house and then, unexplained, walk away. The episode ends with Mulder and Scully's first real kiss.