From its inception, The Big Bang Theory showed that its main characters were enthralled with everything comic books, sci-fi movies, technology and science. Over time, the series became a showcase of pop-culture, reflecting everything from the release of a new iPhone to Star Wars: The Force Awakens finally hitting theaters.
As CBS' hit sitcom became bigger and bigger, it attracted more attention. Sheldon, Leonard, Howard and Raj's obsession with their favorite movies, television shows and creators led to some truly surprising cameos on the series. For example, Star Trek: The Next Generation's Wil Wheaton, Brent Spiner and LeVar Burton all put in appearances on the show, as well as Apple co-founder Steve Wozniak. And that's only the tip of the iceberg.
Now, as The Big Bang Theory approaches its final episode, we take a look back at the series' entire run to determine which five guest stars we never saw coming were its absolute best surprises.
Mark Hamill
Mark Hamill made a surprise guest appearance in Season 11's finale, "The Bow Tie Asymmetry." Famous for playing Luke Skywalker in the Star Wars movies, and for voicing Batman: The Animated Series' Joker, Hamill appeared as himself, and he was widely recognized by the fans at the heart of the show.
What's more, the actor didn't just have a brief cameo. Instead, he appeared throughout the episode as the emotional and hilarious officiant of Sheldon and Amy's wedding.
Stan Lee
Famed comic book writer and co-creator of Marvel's biggest superheroes like Spider-Man, the Fantastic Four and the X-Men, Stan Lee racked up plenty of appearances in the Marvel Cinematic Universe. Still, his cameo on The Big Bang Theory came as a giant surprise.
Stan Lee appeared in the Season 3 episode "The Excelsior Acquisition." In the episode, Lee is scheduled to make an appearance at the guys' local comic book shop, but Sheldon unfortunately misses the event due to an appearance in court. To make it up to him, Penny takes Sheldon right to a grumpy Stan Lee's doorstep.
Bill Gates
Tech genius, millionaire, humanitarian -- the founder, CEO and chief software architect of Microsoft, Bill Gates has more than his hands full. And yet, he still found some time in his busy schedule to guest star onThe Big Bang Theory.
Bill Gates appeared in the Season 11 episode "The Gates Excitation" as himself. In the episode, Penny was tasked with giving a tour of the pharmaceutical company she works at to the Microsoft founder. And, of course, Leonard, Howard and Raj went out of their way to make sure they would "accidentally" bump into one of their heroes.
William Shatner
Since Sheldon, Leonard, Howard and Raj are all fans of Star Trek, William Shatner -- and his iconic turn as Captain Kirk -- has often been the subject of discussion on the series. While George Takei and Leonard Nimoy's voices were both featured on the show, fans wondered if The Big Bang Theory would ever nab the captain of the U.S.S. Enterprise himself.
In the Season 12 episode "The D&D Vortex," Shatner finally put in a brief appearance as a special guest on Wil Wheaton's Professor Proton science show. His presence was meant as a surprise to Sheldon -- and it certainly was. The Star Trek mega-fan vomited all over the actor, making the cameo memorable for more than one reason.
Stephen Hawking
Dr. Stephen Hawking, the brilliant theoretical physicist who changed the way we understand and perceive the universe itself, is Sheldon Cooper's biggest idol. As he puts it, Hawking was one of the only scientific minds who operated on his level.
The Big Bang Theory packed in quite the surprise when Dr. Hawking guest starred on the show in the Season 5 episode "The Hawking Excitation." Not only was he hilarious, but he had such a big impact on Sheldon -- and the series itself -- that he would continue to have a role on the show. Sometimes, only his voice would be heard over the telephone, or he would appear via Skype. His life's work has often been discussed on the show, and his tragic passing was acknowledged in a bonus scene of the Season 11 finale.
The Big Bang Theory airs Thursdays at 8 pm ET/PT on CBS, and will conclude with an hour-long series finale on May 16. The series stars Jim Parsons, Johnny Galecki, Kaley Cuoco, Mayim Bialik, Kunal Nayyar, Simon Helberg and Melissa Rauch.