WARNING: The following list contains spoilers for several DC Comics television shows.

The DC Television Universe is a wondrous place, bringing to the small screen mighty heroes, nefarious villains and gripping moments once limited to the four-color comics page. It's made even more special by its popular and easily recognizable actors, especially when we see them pop up in more than one show. Sometimes, that's done by playing the same character in a new setting. Other times, they're in a new role altogether.

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With DC Entertainment's current opinion being that every live-action franchise exists in one shared multiverse (thanks to CW's "The Flash" having introduced the concept to the Arrow-verse, "Supergirl" and the 1990's "The Flash" series), we've decided not to limit ourselves solely to what's currently on television with this list, but also to include defunct superhero shows and their eye-catching cameos.

15 Christopher Reeve on "Smallville"

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"Smallville" was a different take on the Superman story, focusing on Clark Kent's teenage years before he donned the costume. A casting coup for "Smallville" was the guest appearance of the big-screen Superman himself, Christopher Reeve. Reeve had been paralyzed since 1995 in a horseback-riding accident, but returned to acting and directing in a limited way. Reeve, who died in 2004 at age 52, appeared in two episodes of "Smallville" – his last acting roles – as Dr. Virgil Swann, a wealthy, reclusive scientist.

In the second-season episode, "Rosetta," young Clark Kent (Tom Welling), under the influence of kryptonite, accidentally burned a Kryptonian symbol onto the Kent barn. News coverage of the incident attracts Swann's attention, and he summons Clark to his observatory in New York. He tells Clark of his extensive study of extraterrestrial communications, particularly a transmission from Krypton that he spent many years deciphering with a mathematical key. It is Swann who tells Clark his Kryptonian name, Kal-El, and his belief that Krypton had been destroyed, leaving Clark as the sole survivor.

14 Jack Larson on "Lois and Clark: The New Adventures of Superman"

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ABC's "Lois and Clark: The New Adventures of Superman" from 1993-1997 was modeled on John Byrne's reboot of the Superman titles and character. Featuring Teri Hatcher as Lois and Dean Cain as Clark, the show's focus was more human-scaled than superheroic, choosing to play up the romance between Clark Kent and Lois Lane, while making Superman little more than a disguise for Clark.

In the fourth-season episode, "Brutal Youth," Clark, Lois and Jimmy Olsen (Justin Whalin) investigate an old man who comes to the Daily Planet and dies soon after. Strangely, the old man's ID says he's the same age as Jimmy. The reporters encounter evil scientist Vita Duetsen, who has a device that saps the youth from a body and transfers it to someone else, which she uses to punish men who marry younger women. Jimmy finds the hard way how it works when he's aged from his 20's to his 70's! As a treat for the fans, the older Jimmy Olsen was played by Jack Larson, who had previously played the Jimmy Olsen in 1952's "Adventures of Superman." Sadly, Larson died in 2015 at age 87.

13 Phyllis Coates on "Lois and Clark: The New Adventures Of Superman"

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TV's first Lois Lane was Phyllis Coates, who played the role in the 1951 movie "Superman and the Mole Men," the pilot for the "Adventures of Superman" TV show. Coates played the role during the first season but did not return for the second season, having been replaced by Noel Neill, who previously played Lois Lane in the 1948 "Superman" serial and the 1950 serial "Atom Man vs. Superman."

For "Lois and Clark: The New Adventures of Superman," Coates appeared in the first season finale, "The House of Luthor." In the first half of this two-part story, Lex Luthor (John Shea) connives the Daily Planet's advertisers into withdrawing from the paper, and then buys the Planet on the cheap. He also arranges a bombing at the paper and frames someone else for it, forces out editor Perry White (Lane Smith) and Jimmy (then played by Michael Landes), and proposes to Lois Lane! In the second half of the episode, Coates plays Lois's mother, Ellen Lane, offering Lois some guidance just before she realizes she can't go through with the wedding.

12 Adam West and Frank Gorshin on "Lois and Clark: The New Adventures Of Superman"

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In the second-season "Lois and Clark" episode, "Whine, Whine, Whine," Superman finds out that no good deed goes unpunished when he rescues a musician from being crushed by a falling amplifier at a street fair. His thanks from Calvin Dregg (Jason Carter) is an allegation that Superman injured his hand. Superman soon finds himself facing more bogus claims of injury from several people. Worse, Calvin sues him, magnifying his story from a damaged wrist to a broken arm, and eventually to nerve damage that makes it impossible for him to ever play again. Needing a lawyer, Superman interviews three, including a slick-talking sneak who doesn't impress him. At the trial, Superman prevents a bomb from killing Lois. Calvin quickly tries to claim he was blinded by the blast, prompting Calvin's fed-up wife to reveal his lies, thus helping Superman win the case.

Frank Gorshin, The Riddler from 1966's "Batman," is credited as the sneakier lawyer. He's not the only '60s "Batman" cameo in this episode, though. Adam West, Batman himself, played a flamboyant talk-show host who interviews the greedy musician. Gorshin died in 2005 at age 72, but Adam West is still alive and kicking, having reprised his Batman role through voice acting in the animated "Batman: Return Of The Caped Crusaders" movie.

11 Annette O'Toole on "Smallville"

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In "Superman III," Clark Kent attends his high school reunion in Smallville where he gets reacquainted with his former sweetheart, Lana Lang, now a divorced single mother played by Annette O'Toole. Superman promises to come to a birthday party for Lana's son, but instead of a private moment, the visit turns into a town-wide celebration (as well as an opportunity for the movie's villain to give him synthetic kryptonite, which splits Clark and Superman apart). By story's end, Lana has moved to Metropolis to work as Perry White's secretary at The Daily Planet.

For "Smallville," O'Toole returned to the Superman mythos as Martha Kent, Clark's adoptive mother, taking over the role from Cynthia Ettinger, who played the role in the pilot. O'Toole was with the show for six seasons, as a loving guide to Clark and wife of Jonathan Kent (John Schneider). Martha became a widow when Jonathan died of a heart attack in the fifth season, and later became a Kansas state senator. She was written out of the show when Martha became a U.S. senator and relocated to Washington, D.C., but appeared twice in the ninth season.

10 Terence Stamp on "Smallville"

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Actor Terence Stamp was General Zod in "Superman: The Movie," a renegade Kryptonian exiled to the Phantom Zone with Ursa and Non a short time before the planet was destroyed. In "Superman II," the trio was accidentally released from the Phantom Zone after Superman foiled a terrorist plot by tossing a hydrogen bomb into space. Discovering they had superpowers in Earth's environment, the three battled Superman until he tricked them with a device that depowered them.

For "Smallville," Stamp was the voice of Kal-El's birth father, Jor-El, in 23 episodes through the show's second through 10th seasons. This Jor-El was an artificial intelligence based on the brainwaves of the Kryptonian and was stored in the spacecraft that brought Kal-El to Earth. The A.I. version of Jor-El tried multiple times, sometimes even using harsh methods like mind control, to steer Clark to what he saw as Clark's destiny, becoming Earth's savior. Stamp was uncredited in his first few appearances so as not to tip off the fans.

9 John Wesley Shipp on CW's "The Flash"

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1990's "The Flash" followed the adventures of Barry Allen, played by John Wesley Shipp, a forensic scientist with the Central City Police who is granted speed powers after being struck by lightning and doused with chemicals. In this series, Barry avenges the murder of his brother and continues to fight crime.

The CW's 2014 revival of "The Flash" had several nods to the previous version. One is Shipp, in a recurring role as Barry's dad, Henry, in the first and second seasons of the new show. Here, the tragedy that motivates Barry Allen (Grant Gustin) occurs well before he gets speed powers: his mother Nora is murdered by the Reverse-Flash, and Henry is imprisoned for it. Henry was eventually cleared of the murder and released from prison, but was murdered by the villain Zoom in season two. Shipp appeared in the season finale in a new guise: as Jay Garrick, the Flash of Earth-3, who had been held captive by Zoom throughout the year.

8 Amanda Pays on CW's "The Flash"

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On 1990's "The Flash," Christina "Tina" McGee (Amanda Pays) was a scientist at S.T.A.R. Labs.  After the accident that turned Barry Allen into a speedster, Tina helped him get a handle on his new abilities. She provided a prototype of a deep-sea diver's suit that Barry modified for use as his superhero uniform, and aided The Flash on his cases.

In the 2014 revival, Tina leads competing tech firm Mercury Labs. In the episode "The Man in the Yellow Suit," The Flash appeals to her to help catch the Reverse-Flash with a tachyon prototype invented by her company as bait. In "All-Star Team Up," Tina is targeted by Brie Larvan, a disgruntled former Mercury Labs tech who attacks Tina with robotic bees. And in "The Reverse-Flash Returns," the villain nearly murders Tina before The Flash stops him.

Amanda Pays played the character on both shows, but in very different capacities. In the 1990s version she was a possible love interest for the Flash, while the CW incarnation, she serves solely as an occasional source of advice and help.

7 Mark Hamill on CW's "The Flash"

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Mark Hamill indulged his villainous side as The Trickster in 1990's 'The Flash." He appeared in the episode "The Trickster" as James Jesse, a psychotic man stalking private investigator Megan Lockhart. The Flash captures him, but he escapes police custody and takes on the guise of The Trickster, bent on "rescuing" Lockhart from The Flash and terrorizing Central City. Hamill returns in "The Trial of The Trickster," the last episode of the original run.

In the 2014 show, Hamill was The Trickster again, serving 20-plus years in Iron Heights Prison for his crimes. In a story reminiscent of a G-rated version of "Silence Of The Lambs," Barry Allen and Detective Joe West (Jesse L. Martin) seek his help in catching a new Trickster now terrorizing the city using his gimmicks, such as flooding a park with bombs disguised as toys. Jesse feigns anger that there's a copycat and escapes vowing to kill him, but it all ends up being a ruse as the impostor is revealed to be Jesse's son.

Before his return to "The Flash," Hamill was the voice of The Joker in "Batman: The Animated Series" and in the "Batman: The Killing Joke" film that adapted the classic graphic novel.

6 Alex Désert on CW's "The Flash"

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Season three of "The Flash" revival is based heavily on the "Flashpoint" crossover storyline from the comics, in which Barry Allen time travels to the past and prevents his mother's murder at the hands of the Reverse-Flash, instead of letting things take their natural course. Returning to his own time, Barry finds reality is changed in several ways. Both of his parents are alive, his father figure, Joe, is now a drunk, and Iris's brother, Wally, is now a speedster calling himself The Flash while Iris supports him. Another difference in the new reality was that Captain David Singh was no longer in charge of the Central City Police. Instead, Barry answered to a stranger, Captain Julio Mendez.

Sharp-eyed viewers of the season three premiere episode, "Flashpoint," recognized Mendez as another familiar face from the 1990 "Flash" series: Barry Allen's fellow scientist and lab partner Julio Mendez, again played by Alex Désert, albeit for one episode.

5 Helen Slater on CW's "Supergirl"

Helen Slater as Supergirl

TV's "Supergirl" follows young Kara Zor-El, who is sent to Earth by her parents from the destruction of Krypton with a mandate to protect her baby cousin, Kal-El. However, her spacecraft goes off-course and into the Phantom Zone for a time. When she arrives on Earth, Kara is now a pre-teen about 10 years younger than Kal instead of 12 years older. Superman arranges for her to live with the Danvers family, and she doesn't embrace her heroic identity until years later, when she has to rescue her adoptive sister's disabled airplane.

Kara's adoptive mother on TV's "Supergirl," scientist Eliza Danvers, is none other than Helen Slater, the big-screen Supergirl from the spinoff to the Christopher Reeve "Superman" movies. In that film, young Kara comes to Earth to retrieve the power source for Argo City and battles a witch who wants to keep it for herself. Slater also previously did a turn on "Smallville" as Kara's mother, Lara-El, for two episodes in 2007 and once in 2010.

4 Dean Cain on "Supergirl"

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For Supergirl's TV dad, Jeremiah Danvers, producers got a real Superman: Dean Cain, the lead from "Lois and Clark: The New Adventures of Superman." Superman leaves the young Kara in the care of Jeremiah and Eliza Danvers, but the handoff doesn't escape the attention of the Department of Extra-Normal Operations. In the first season episode, "Livewire," we learn that at one point in her youth, after Kara had used her powers in public, DEO agents came to the Danvers home to take her away. Jeremiah makes a deal to work for them instead if they'll leave Kara alone.

In "Manhunter," we learn why J'onn J'onnz (David Harewood) has been impersonating DEO director Hank Henshaw: The real Henshaw led a mission 10 years ago in Peru to kill the Martian. But J'onn saves Jeremiah from being bitten by a snake, and the two share stories of being parents. Henshaw, bent on killing J'onn, won't listen to Jeremiah's pleas that J'onn is not a threat, and they fight. Henshaw stabs Jeremiah, who pushes Henshaw off a cliff, so J'onn takes Henshaw's place thereafter.

3 Laura Vandervoort on "Supergirl"

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If one guest Supergirl is good for the "Supergirl" TV series, then two is even better. "Smallville's" Supergirl, Laura Vandervoort, was in three first-season episodes as Brainiac-8, a sentient computer being who was imprisoned in Fort Rozz, the holding facility for Krypton's criminals. Fort Rozz went into the Phantom Zone when Krypton exploded, dragging Kara's escape pod along and delaying her arrival on Earth. Brainiac-8 took control of Kara's escape pod and started Fort Rozz's engines, allowing it to escape the Zone.

Taking the name Indigo, Brainiac-8 infiltrated computer systems on Earth. Indigo formed an alliance with Non, leader of Kryptonian escapees from Fort Rozz, with the goal of wiping out human life. She ultimately enthralled all of National City with a brainwave signal that threatened to make everyone's heads explode. J'onn J'onnz destroyed Indigo and Supergirl thwarted her plot, sacrificing her life by flying Fort Rozz into space. Fortunately for her, sister Alex (Chyler Leigh) rescued her with Kara's escape pod.

2 Lynda Carter on "Supergirl"

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Supergirl is as excited as any fan when the President of the United States comes to National City, especially because it's TV's Wonder Woman herself, Lynda Carter, in the role! Carter was the lead on "Wonder Woman" in 1975, which was on ABC in its first season and on CBS in its second and third seasons.

In the second-season episode of Supergirl, "Welcome to Earth," President Olivia Marsdin visits the Department of Extra-normal Operations headquarters. She is in town to sign an executive order granting amnesty to aliens on Earth, triggering attacks by an alien who doesn't trust that amnesty means anything good for her kind. Carter and the writers clearly had fun with the role, slyly saying to Kara "you should see my other jet" when Supergirl gushes about Air Force One. The line is a clearly a nod to the fans who remember Wonder Woman's invisible jet.

1 BONUS: George Reeves as Superman on "I Love Lucy"

George Reeves as Superman on I Love Lucy.

Our final entry in this list breaks away from the original goal of discussing cameos in DC TV shows, but it's a cameo that's too fun not to discuss.

In an episode of "I Love Lucy," a simple wish that Superman would appear at Little Ricky's birthday party (thus showing up another popular kid whose party is on the same day) becomes a day of shenanigans when Lucy can't deliver on her promise. In order to save the day, she resolves to appear as Superman herself, in a makeshift costume making use of a football helmet to hide her red hair. Superman does make the party, to the delight of Little Ricky and his friends, but Lucy misses it all because she's stuck on the ledge outside her apartment window, her cape snagged on a pipe! Of course, Superman comes to her rescue, not just from falling off the ledge, but also from an angry Ricky who yells, "Of all the crazy things you've done in the 15 years that we've been married – "

Superman interrupts, "Mr. Ricardo! You mean to say you've been married to her for 15 years?"

"Yeah, 15 years!"

"And they call me Superman!"

All this happened in the episode "Lucy and Superman," excerpted here on YouTube, featuring George Reeves in character as Superman.

It's fun to spot popular actors in cameos. What are your favorite examples? Let us know in the comments!