Warner Bros. Television controls the online rights for the show. Full episodes are available at tv.com, while short clips and recently aired full episodes are available on cbs.com. In Canada, recent episode(s) and pictures are available on CTV.ca. Additionally in Canada, the first six seasons are available for streaming on Bell Media's CraveTV. After the show has aired in New Zealand the shows are available in full online at TVNZ's on demand web service.
One of the recurring plot lines is the relationship between Leonard and Penny. Leonard becomes attracted to Penny in the pilot episode and his need to do favors for her is a frequent point of humor in the first season. Meanwhile, Penny dates a series of muscular, attractive, unintelligent, and insensitive jocks. Their first long-term relationship begins when Leonard returns from a three-month expedition to the North Pole in the season 3 premiere. However, when Leonard tells Penny that he loves her, she realizes she cannot say it back. Both Leonard and Penny go on to date other people; most notably with Leonard dating Raj's sister Priya for much of season 4. This relationship is jeopardized when Leonard comes to falsely believe that Raj has slept with Penny, and ultimately ends when Priya sleeps with a former boyfriend in "The Good Guy Fluctuation".
Leonard's mother Beverly visits, and he is thrilled when she takes an interest in his work. However, when he learns that she is doing research on him for a new parenting book, he is furious. He prepares to tell her how she let him down over the years, but instead chooses to accept and forgive her, and forgive himself for holding onto that resentment for so long. Beverley is touched that he forgave her and they share a genuine hug. Meanwhile, Anu gets a job offer in London. Raj prepares to fly out and propose to her, ready to follow her if she accepts. Howard stops Raj at the airport, saying there is a better match out there for him, convincing Raj to return home. Stuart and Denise are bothered by her creepy roommate Mitch, while Bernadette and Howard are tired of Denise constantly staying overnight with Stuart at their house. Stuart says he is ready to move in with Denise, and they say they love each other while convincing Mitch to move out.
Turning to total viewers, “Manifest” was the biggest gainer overall, on average adding 6.1 million viewers per episode. ABC’s “The Good Doctor” was second with an average viewer lift of just under 6 million. “New Amsterdam” (5.2 million viewer lift) and “A Million Little Things” (3.5 million viewer lift) were also among the top gainers in this measure, as was ABC’s “Whiskey Cavalier” (3.2 million viewer lift).
^ Kondolojy, Amanda (November 8, 2013). "Thursday Final Ratings: 'Elementary', 'Scandal', 'The Vampire Diaries', & 'The Big Bang Theory' Adjusted Up; 'The Voice', 'The Millers', 'Sean Saves the World', 'Parenthood', & 'The Michael J Fox Show' Adjusted Down". TV by the Numbers. Archived from the original on November 8, 2013. Retrieved November 8, 2013.
Cosmologist Stephen Hawking made a short guest appearance in the fifth-season episode; in the eighth season, Hawking video conferences with Sheldon and Leonard, and makes another appearance in the 200th episode. In the fifth and sixth seasons, NASA astronaut Michael J. Massimino played himself multiple times in the role of Howard's fellow astronaut. Bill Nye appeared in the seventh and twelfth seasons.
Raj meets with the woman his father set up for him, Anu. Her commanding personality and practical approach to marriage startle Raj but he agrees to go ahead with it. Howard criticizes him but apologizes; he just never expected Raj to give up on romance. Raj admits this to Anu so she proposes to him, and he says yes. Raj also sends out a relationship questionnaire to Leonard and Penny, which soon reveals that Penny does not think she ever wants kids, upsetting Leonard, who would like to have kids someday, and their friends selfishly badger them on the issue. After telling her father Wyatt who is also upset, Leonard tells Penny that he'll be okay with it, as he never thought he'd have her in his life. To get his mind off of it, Penny rents Leonard a Batmobile to drive in and have fun with for the day. Meanwhile, Stuart starts bringing home Denise at night, which is very uncomfortable for Howard and Bernadette. When they also try to have time to themselves, they wake up their kids.
Two physicists from Chicago, Dr. Campbell and Dr. Pemberton, accidentally prove Sheldon and Amy's super-asymmetry theory. This puts them on track for a Nobel Prize, but Leonard points out they tend to go to the experimental scientists who prove a concept instead of theorists like Sheldon who come up with it. Meeting with them, they tell Sheldon they are willing to share the Prize but only three names can be on the submission, and they want to cut Amy out. Amy does not want to prevent Sheldon's lifelong dream, but he goes to President Seibert who agrees to fight for Amy on their behalf. Meanwhile, Bernadette's drug has been approved and she wants to promote Penny to lead the marketing team. Penny resists, knowing Bernadette's attitude at work and doubting her own inexperience to take the responsibility, but Bernadette uses reverse-psychology to get Penny to take the job. Penny mirrors Bernadette's commanding style to get the other employees in line, impressing Bernadette.
Filming proceeds smoothly – makeup artists do touch-ups, a prop master replenishes the pickle supply – with humor and camaraderie between takes. When a reshoot is needed because Simon Helberg, who plays Howard, held his napkin in the wrong hand, Kaley Cuoco, who plays Penny, lifts her food box high and mockingly asks, “Does it go over my head? I don’t remember.”
Melissa Rauch as Bernadette Rostenkowski-Wolowitz (recurring season 3, starring seasons 4–12): A young woman who initially is a co-worker at The Cheesecake Factory with Penny to pay her way through graduate school, where she is studying microbiology. Bernadette is introduced to Howard by Penny; at first, they do not get along, apparently having nothing in common. They date and later get engaged and married. Although generally a sweet and good-natured person, Bernadette has a short fuse and can be vindictive and lash out when provoked.
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.
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".
When Leonard receives a braggy Christmas letter from his brother, he feels bad about how little he and Penny have achieved in the past year. Leonard wonders if he and Penny should be thinking about the next step after two years of marriage, like buying a house or having a baby. Penny tells him she wants to do all those things someday, but she has a bunch of stuff she would like to do first.