As the theme of the show revolves around science, many distinguished and high-profile scientists have appeared as guest stars on the show. Famous astrophysicist and Nobel laureate George Smoot had a cameo appearance in the second season. Theoretical physicist Brian Greene appeared in the fourth season, as well as astrophysicist, science populizer, and physics outreach specialist Neil deGrasse Tyson, who also appeared in the twelfth season.
The show originally centered on five characters living in Pasadena, California: Leonard Hofstadter and Sheldon Cooper, both physicists at Caltech, who share an apartment; Penny, a waitress and aspiring actress who lives across the hall; and Leonard and Sheldon's similarly geeky and socially awkward friends and co-workers, aerospace engineer Howard Wolowitz and astrophysicist Raj Koothrappali. Over time, supporting characters were promoted to starring roles, including neuroscientist Amy Farrah Fowler, microbiologist Bernadette Rostenkowski, physicist Leslie Winkle and comic book store owner Stuart Bloom.
After the strike ended, the show was picked up for a second season, airing in the 2008–2009 season, premiering in the same time slot on September 22, 2008. With increasing ratings, the show received a two-year renewal through the 2010–11 season in 2009. In 2011, the show was picked up for three more seasons. In March 2014, the show was renewed again for three more years through the 2016–17 season. This marked the second time the series gained a three-year renewal. In March 2017, the series was renewed for two additional seasons, bringing its total to 12, and running through the 2018–19 television season.
According to Steve Holland who is the show-runner starting with this season, Wil Wheaton will return in this season since his absence from Season 10. And this has confirmed to be true since it has been stated that he reappeared in "The Proton Regeneration" (S11E6), along with Professor Proton, their first appearances since "The Opening Night Excitation" (S9E11) and "The Celebration Experimentation", respectively and a span of 43 episodes and a span of 37 episodes, respectively. Wil also returned in the 15th episode. Wil will also return for Sheldon and Amy's Wedding.
However, no legal action was required to end production of the other show: as soon as it became known that the show was unlicensed, the actors quit and the producers canceled it. Dmitriy Tankovich (who plays Leonard's counterpart, "Seva") said in an interview, "I'm upset. At first, the actors were told all legal issues were resolved. We didn't know it wasn't the case, so when the creators of The Big Bang Theory started talking about the show, I was embarrassed. I can't understand why our people first do, and then think. I consider this to be the rock bottom of my career. And I don't want to take part in a stolen show".