Tonight, the TV show Lost hits the milestone of its 100th episode. Few sci-fi TV shows manage to reach 100 episodes—for example, even the long-running Quantum Leap only made it to 96—but when they do, we always look forward to it. The cast and crew will often have a big party, and the viewers are usually given a special episode to enjoy, something with stunt casting or major plot developments.

But that wasn't always the case. The 100th episode of The Twilight Zone was Ray Bradbury's "I Sing the Body Electric" (May 18, 1962) but nothing special was done for it, possibly because the program was an anthology show. Neither were the 100th episodes of Bewitched (March 9, 1967) or I Dream of Jeannie (January 6, 1969) anything special. The 100th serial of the original Doctor Who, "Stones of Blood," (October-November 1978) almost had a scene with a cake to celebrate the Doctor's 751st birthday, but in the end the show runners felt that it would be too self-indulgent to acknowledge the milestone.

Times have changed, however, and in the 1990s, sci-fi series began to offer special 100th episodes. Here, in chronological order, are 16 of sci-fi's most memorable 100th episodes:

Star Trek: The Next Generation
"Redemption, Part One"
June 17, 1991

To restore his family honor, security chief Worf resigns his Starfleet commission and leaves the Enterprise to fight on the side of Gowron in a Klingon civil war.
How Was It Special? The episode continued an ongoing saga about Worf and his family, and ended with the shocking return of Denise Crosby as a Romulan officer, making viewers wonder if her deceased former character of security chief Tasha Yar still had a part to play.
Was It Special Enough? Absolutely. The previous season had ended on a major cliffhanger, and "Redemption" continued the tradition of giving the audience something to discuss and debate over the summer hiatus.

Star Trek: Deep Space Nine
"The Ship"
October 7, 1996

Captain Sisko tries to claim the right to take a crashed Jem'Hadar ship, while an enemy Vorta named Kilana negotiates with him for its return.
How Was It Special? It wasn't. Nothing was done to note the milestone.
Was It Special Enough? Although it was a good episode with some notable revelations at the end, it's simply not memorable as a 100th episode.

Highlander
"Revelation 6:8"
February 15, 1997

The evil Kronos brings the Four Horsemen of the Apocalypse together to wreak mayhem, and Duncan MacLeod has to stop them.
How Was It Special? The Four Horsemen of the Apocalypse are Immortals, just like the Highlander. How cool is that?
Was It Special Enough? Yes, although calling this the 100th episode in the United States is a bit problematic. Although it was the 100th episode produced, and the 100th shown in Europe and Canada, due to odd scheduling the episode was the 99th one broadcast in the U.S.

The X-Files
"Unusual Suspects"
November 16, 1997

This flashback episode set in 1989 shows how the Lone Gunmen first came together to expose conspiracies.
How Was It Special? Viewers always enjoyed seeing guest stars Bruce Harwood (as Byers), Dean Haglund (as Langly), and Tom Braidwood (as Frohike) show up, as it usually promised a more humorous episode. Also, Richard Belzer made a cameo appearance as detective John Munch, theoretically uniting The X-Files with the worlds of Homicide: Life on the Street and Law & Order: Special Victims Unit.
Was It Special Enough? Yes. Ever since the Lone Gunmen conspiracy theorists had been introduced in season one, viewers wanted to know more about how these three eccentric characters (a formal federal employee, a nerd computer hacker and a voyeuristic audio-video technician) had chosen to work together.

Babylon 5
"The Ragged Edge"
April 8, 1998

Michael Garibaldi's mission to the Drazi Homeworld is put in jeopardy when he has a relapse into alcoholism, while G'Kar discovers that his people, the alien Narn, now consider him a religious figure.
How Was It Special? Well, it wasn't.
Was It Special Enough? Not really. The fifth season of the show is generally considered the weakest, and this episode did very little other than move the story arc forward.

Star Trek: Voyager
"Timeless"
November 18, 1998

Future versions of Commander Chakotay and Ensign Harry Kim try to avert a quantum slipstream disaster that destroyed starship Voyager and killed everyone on board.
How Was It Special? LeVar Burton, who directed the episode, also appeared as a future version of his popular Next Generation character, Geordi La Forge.
Was It Special Enough? Yes. Star Trek always has a lot of fun with time travel, and viewers always enjoy seeing alternate versions of their beloved characters. Furthermore, because the quantum slipstream experiment did cut years off their trip, Captain Janeway notes in her personal log that they've passed a milestone in their journey home. It's no longer a question of if they will get home, but when.

Hercules: The Legendary Journeys
"Once Upon a Future King"
April 26, 1999

A Camelot tie-in in which the magician Merlin sends the tyrannical King Arthur back in time to learn the ways of peace from Hercules.
How Was It Special? The Camelot connection could be considered special, but Hercules interacted with other mythical and historical figures throughout the run.
Was It Special Enough? Most viewers enjoyed the episode, but nothing made it stand out for all that it was the 100th episode.

Xena: Warrior Princess
"Lyre, Lyre, Hearts on Fire"
January 17, 2000

Xena holds a battle of the bands to determine the winner of an enchanted lyre.
How Was It Special? It was a musical!
Was It Special Enough? A fun episode filled with humor, love, and kick-ass action, it reiterated what helped make the show so popular and gave the audience a chance to relax after some more stressful episodes.

Buffy the Vampire Slayer
"The Gift"
May 22, 2001

In this fifth season finale, Buffy sacrifices her life to close a demon portal and save the life of her sister Dawn.
How Was It Special? Buffy died, and apparently stays dead.
Was It Special Enough? Yes. With the series ending its run on the WB Network and moving to UPN, Buffy's death (however short it would turn out to be) brought the season to a powerful conclusion.

Stargate SG-1
"Wormhole X-Treme!"
September 8, 2001

The SG-1 team investigates a new low-budget sci-fi TV show that seems to be based on their real experiences.
How Was It Special? The episode featured cameos of many crew members, including director Peter DeLuise and producers Robert C. Cooper, Joseph Mallozzi, and N. John Smith, who acted like the clueless Hollywood types that fans often fear are ruining their favorite SF programs.
Was It Special Enough? Absolutely. The "show within a show" poked fun at the conventions of science fiction television, but with an obvious respect and love for the genre and their fans.

Charmed
"Centennial Charmed"
January 19, 2003

Wanting to win Phoebe's heart back, half-demon Cole casts a spell that alters reality, sending Paige into a world where her sisters don't remember her and they no longer have the Power of Three to defend themselves.
How Was It Special? Viewers enjoyed seeing the alternate universe versions of the Halliwell sisters.
Was It Special Enough? Yes. The episode illustrated the powerful bond among the sisters and brought the Cole-Phoebe love story to a sad but necessary conclusion.

Angel
"You're Welcome"
February 4, 2004

Cordelia Chase awakens from her coma and helps Angel get back on track to fight evil.
How Was It Special? Cordelia returns and they show old footage featuring Doyle, a pivotal character who had died in first season.
Was It Special Enough? Yes. The episode showcased a popular character and hearkened back to the beginning, when Angel, Cordelia, and Doyle were the only three characters on the show.

Andromeda
"Pride Before the Fall"
January 21, 2005

Beka's new boyfriend, Peter, leads the crew into danger, as all is not what it appears to be on the surface.
How Was It Special? The episode opened with a welcome message from Kevin Sorbo and included almost two minutes worth of outtakes at the end as a thank you to the viewers.
Was It Special Enough? The episode's story resolved some questions from the show's mythology and fans appreciated the outtakes.

Smallville
"Reckoning"
January 26, 2006

Clark reveals his secret to Lana, which through a series of events leads to her death. Clark appeals to his birth father Jor-El, who restores Lana to life by turning time backward, but at the sacrifice of another's life.
How Was It Special? It featured the death of Clark's father, Jonathan Kent.
Was It Special Enough? Yes. By killing off Clark's father but leaving his mother alive, the show moved the Superman mythology into uncharted territory.

Alias
"There's Only One Sydney Bristow"
April 26, 2006

Sydney has to cut short her maternity leave to rescue her good friend, reporter Will Tippin, who has been abducted by her rival Anna Espinosa.
How Was It Special? The audience got to see the return of actress Gina Torres as arch-villain Espinosa.
Was It Special Enough? Yes. Before this episode, Sydney struggled with her concern over the danger she brought into the lives of her friends. But by the end of the episode we come to realize that her friends are better off for knowing her.

Stargate Atlantis
"Enemy at the Gate"
January 9, 2009

The Atlantis crew must stop a powerful new Wraith ship from reaching Earth.
How Was It Special? It ended up being the last episode of the series.
Was It Special Enough? Yes. Although the show got cut off in its prime, the show runners managed to give the audience a satisfying conclusion to the series. The heroes save the day and show that they're willing to make the sacrifices we'd expect of them, but it's clear that their job isn't done yet. After all, one day that cloak might fail and the residents of San Francisco will notice the city of Atlantis sitting just past the Golden Gate Bridge...