It's hard to believe The Big Bang Theory has been on TV for ten whole seasons. What's even more amazing is that the show has continued to be one of the highest rated sitcoms in television year after year. While it's difficult to imagine the show's star geeks as anything but who they are on the show, they've all had rich histories in Hollywood. Like most successful actors, they have some pretty awkward, embarrassing or surprising roles from their past that can only be found by the most skilled internet diggers who search into the wee hours of the morning.

While you may have heard of some of these past roles of your favorite Big Bang Theory cast members, there are some that are downright shocking. Thankfully, we have wonderful internet tools like YouTube that help us uncover these mysterious early acting gigs for our viewing pleasure. You'll be surprised how many of these Big Bang Theory stars have slipped right under our noses in some of our favorite TV shows over the years like 7th Heaven, Roseanne and even Charmed.

While there are more than just 15 great roles from these actors' pasts, here are the top 15 you simply must know about.

15. Kaley Cuoco on 8 Simple Rules

Landing one of the lead roles on 8 Simple Rules was what really put Kaley Cuoco on the map as a successful actress in Hollywood. Kaley joined a star studded cast consisting of John Ritter, Katey Sagal and Amy Davidson. The show followed Kaley's character and her siblings as they grew into adulthood, and the impact it had on their anxious father. Unfortunately, in the middle of the show's run, John Ritter tragically passed away suddenly, forever changing the dynamic of the show. Although John Ritter was no longer on the show, the sitcom went on for a couple more seasons before getting cancelled. While most of the characters remained on the show (including Kaley's character), it really was never the same since John Ritter's tragic passing.

14. Johnny Galecki on National Lampoon's Christmas Vacation

Today, we know Johnny Galecki best as Leonard Hofstadter, the smart and awkward geek on Big Bang Theory. But before Johnny made it big on the show, he scored some pretty decent TV and movie roles over the years. One of the earliest roles of Johnny's career was in beloved cult classic National Lampoon's Christmas Vacation. In 1999, the 14-year old served as Rusty, Chevy Chase's eager teenage son. While Johnny did a great job playing Rusty, the role was played by different actors in just about every different film of the franchise. Rusty has also been played by actors like Anthony Michael Hall and even Ethan Embry in later years.

13. Jim Parsons on Garden State 

It's hard to imagine Jim Parsons as anyone else but the socially awkward Sheldon Cooper on The Big Bang Theory, but he did have a few memorable roles prior to his big break. While Garden State is remembered as the film that made the band The Shins famous and what really put Zach Braff on the map, there were some seriously hilarious characters in the film that made it the awesome movie it was. Jim Parsons played the young boyfriend of Zach Braff's character's pot-smoking mom. Due to the fact that his character worked at Medieval Times, we watched him awkwardly walk around in a clunky iron suit as he ate his bowl of lucky charms. While it was a small role, it really was one of the best scenes in the entire movie.

12. Kaley Cuoco on Charmed

As we mentioned before, Kaley was no stranger to the small screen before playing Penny on The Big Bang Theory. In 2006, Kaley appeared as Billie Jenkins on WB's Charmed. The show circled around a family of witches played by Alyssa Milano, Holly Marie Combs  and Rose McGowen. Kaley's character astro-projects and can change the course of fate giving her what they call the "ultimate power". Initially, Kaley's character was supposed to serve as a starting point for a spin-off of Charmed, but it was later cancelled by the network. Apparently, the character of Billie wasn't too well received by critics, and Kaley's acting was a big part of it. Little did they know Kaley would go on to be on one of the most successful sitcoms ever.

11. Simon Helberg in A Cinderella Story

Simon Helberg plays the goofy, ever-dorky Howard Wolowitz on The Big Bang Theory. But before he was spouting out math equations, quoting Star Trek and dating (and marrying) Bernadette, Simon had a small role in the film, A Cinderella Story, starring Hillary Duff and Chad Michael Murray. In the movie Simon plays a, you guessed it: a total nerd. Throughout the film, Simon's character, Terry chases after Hillary Duff's character around school as he continues to live in his own little world. Oddly enough, the character of Terry isn't too far off from The Big Bang Theory's Simon Helberg. Interesting, very interesting.

10. Kunal Nayyar on NCIS

Unlike some of the rest of the cast, Kunal Nayyar's role on The Big Bang Theory was one of his first real TV roles ever. When he got cast as Rajesh Koothrappali on the hit sitcom, he only had a couple of TV credits to his name, with one of them being his co-star role on the show, NCIS. Kunal appeared on season 4, episode 9 of NCIS, as an Iraqi terrorist alongside the show's leading man, Mark Harmon. Apparently, Kunal's role on NCIS earned him his SAG card, allowing him to take on more credible roles in Hollywood. Who knows if he would have ever landed a huge show like The Big Bang Theory if he had never worked on a show like NCIS.

9. Mayim Bialik on Blossom

Sure, Mayim Bialik has really made her mark on The Big Bang Theory as neurobiologist, Amy Farrah Fowler; but we first came to know and love the actress when she was the star of her own sitcom back in the '90s, Blossom. The character of Blossom touched the hearts of young girls all over the country as they watched her try to navigate through life being raised by three males. She also made quite a splash in the fashion department with her floral prints and statement hats that in many ways, basically define the '90s. While we really do love Mayim Bialik on The Big Bang Theory, we really can't help but miss the good ol' days of Blossom.

8. Johnny Galecki on Roseanne

Like we said, The Big Bang Theory wasn't Johnny Galecki's first time on a big sitcom. In the '90s, Johnny played innocent teen, David on the sitcom, Roseanne. Roseanne starred comedian, Roseanne Barr and John Goodman, and was America's favorite sitcom about a family in middle America just trying to get by. The show's middle daughter, Darlene (played by Sara Gilbert) dates Johnny's character for most of the show. The two characters basically grow up together, allowing David to really become part of the family. David plays a completely different role on The Big Bang Theory as Leonard, but the same kind, caring heart can be seen in his character as David.

Fun fact: Johnny Galecki and Sarah Gilbert had a bit of a reunion on The Big Bang Theory when Sara appeared in a handful of episodes as her character, Leslie Winkle.

7. Melissa Rauch on Kath and Kim

Before Melissa Rauch played the adorable smarty pants, Bernadette on The Big Bang Theory, she had a few small roles on TV and film. Most notably, she had a small role on the hilarious, Kath and Kim. While the show isn't super well-known here in the United States, Kath and Kim was a huge hit in Australia and featured a mother (Molly Shannon) and daughter (Selma Blair) as they navigate through their many dysfunctional relationships. Melissa Rauch played Tina on the show and was really able to portray her comedic chops. While the show had its funny moments, Kath and Kim was quickly cancelled after just one season.

6. Mayim Bialik in Beaches

Everyone knows that if they're in need of a good cry, cuddling up on the couch and watching the movie, Beaches will do just the trick. While Mayim only appeared in a small portion of the film as the child version of Bette Midler's character, she really did make an impression. Casting her as the young Bette Midler was pretty brilliant casting if you ask us-- she even had the cute, red curly hair to match. If you took anything away from the film, you learned to appreciate good friends and that the song, 'You Are the Wind Beneath My Wings' can make you crumble into a blubbering mess within the first couple measures.

5. Kevin Sussman in Wet Hot American Summer

We all know Kevin Sussman as the hilarious comic book store owner on The Big Bang Theory, but it wasn't his first acting gig. He may not have had the biggest part in the film, but Kevin Sussman actually starred in the incredible cult classic, Wet Hot American Summer. The film follows a cast of camp counselors on the last day of camp as they attempt to tie loose ends on the very last day of camp. Kevin played Steve, the awkward outcast who only spoke in an annoying robot voice and had supernatural powers. Kevin reprised his role as Steve in the Netflix reboot of Wet Hot American Summer, but we can't help but think of him as The Big Bang Theory's epic comic store owner.

4. Jim Parsons on Judging Amy

Have a strange feeling that you've seen Jim Parsons long before The Big Bang Theory? If you were a fan of the dramedy, Judging Amy, then you definitely have seen the actor before. Jim Parsons was on a total of seven episodes of Judging Amy as Rob Holbrook. Judging Amy was on the air from 1999-2005 starring Amy Brenneman and Tyne Daly. The show followed three generations of women as they go through life together in Hartford, Connecticut. It doesn't seem like Jim Parsons made much of an impact on the show, as there isn't much about said about it. However, you can find clips from his Judging Amy episodes on YouTube fairly easily.

3. Kaley Cuoco on 7th Heaven

Before she graced the small screen as Penny on The Big Bang Theory, Kaley Cuoco played many small roles on a handful of dramas and sitcoms back in the '90s and late '00s. We really had to wrack our brains for this one, but we do faintly recall Kaley playing a small part of the wholesome family sitcom, 7th Heaven. While Kaley's character on the show wasn't really that important to any of the main character's storylines, she did serve as a source of drama for Simon Camden, played by David Gallagher. Kaley played Lynn, the girlfriend of one of Simon's friends. However, Simon develops a crush on Lynn, forcing him to deal with his feelings and possibly jeopardize his friendship. In true 7th Heaven fashion, Simon learns a valuable lesson about love and friendship.

2. Simon Helberg on Mad TV

While most people only recognize Simon Helberg as one of the main dweebs of The Big Bang Theory, true fans remember his stint on the hilarious sketch show, Mad TV in 2002-2003. What's funny, is that Simon oftentimes spoofed kitschy sitcoms-- which is quite ironic given that he's on one of the biggest sitcoms ever, The Big Bang Theory. Simon wasn't the only Mad TV cast member to make it big. Similar to his character on The Big Bang Theory, Simon loved playing the "goofy guy characters"-- and he did it pretty well. Little did we know Simon would go on to have a pretty awesome career in television. Simon rubbed shoulders with other actors and comedians that ended up making it big after their time on Mad TV, like Key and Peele, Alex Bornstein, Taran Killam and Ike Barinholtz

1. Melissa Rauch on True Blood

Melissa makes this list yet again with her role as Melissa on HBO's True Blood. Melissa appeared on the dramatic vampire series in its third season as character, Summer, who was deeply in love with the character Hoyt Fortenberry (Jim Parrack). Throughout her six episodes on the show, Summer continued to fight for Hoyt's affection, although he ultimately chose Jessica Hamby (Deborah Ann Woll) and ended up marrying her. Shortly after her stint on True Blood, Melissa landed her role on The Big Bang Theory where she had much more lasting success. While we always will think of Melissa as the sexy yet geeky doctor on our favorite sitcom, we can't help but think of her fondly as the sexy Summer on True Blood.

Sources: usmagazine, tv, bio, tvinsider, wikipedia