Johnny Galecki as Leonard Hofstadter:[42] An experimental physicist with an IQ of 173, who received his PhD when he was 24 years old. Leonard is a nerd who loves video games, comic books, and Dungeons & Dragons. Leonard is the straight man of the series, in which he shares an apartment in Pasadena, CA, with Sheldon. Leonard is smitten with his new neighbor Penny when they first meet, and they eventually marry.

There was an unaired pilot which CBS did not broadcast. In March 2017, CBS announced that the series was renewed for two more years, leading the show to a total of 12 seasons.[1] The twelfth season premiered on September 24, 2018,[2] and served as the series' final season.[3] During the course of the series, 279 episodes of The Big Bang Theory aired over 12 seasons.
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.[209] 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".[210]
Sara Gilbert as Leslie Winkle (recurring season 1, starring season 2, guest seasons 3, 9):[47][48][49] A physicist who works in the same lab as Leonard. In appearance, she is essentially Leonard's female counterpart and has conflicting scientific theories with Sheldon. Leslie has casual sex with Leonard and later Howard. Gilbert was promoted to a main cast member during the second season but resumed guest star status because producers could not come up with enough material for the character.[47] Gilbert returned to The Big Bang Theory for its 200th episode.[50]

While Johnny Galecki is putting down the comic books for good as he says goodbye to experimental physicist Leonard Hofstadter, he's poised to return to another iconic role one more with the expanded season two order of Roseanne spinoff The Conners coming to ABC this fall. Galecki reprised the role of David Healy, ex-husband to Sara Gilbert's Darlene, for a handful of episodes in the show's first season and the door was definitely left open for more. Despite Galecki telling TV Line in February that he wouldn't consider jumping in full-time—"I think I need a little bit of space from being a series regular again," he said—he's hoping to "visit as much as possible." And Gilbert, who serves as EP on the series, remains hopeful as well. "I know he loves doing the show. I love having him. We have a magical time together," she told the outlet in January. "I would anticipate that he would do some [episodes] but I don't want to speak for him."
The Big Bang Theory is an American comedy television series created and executively produced by Chuck Lorre and Bill Prady. Like the name of the series itself (with the exception of the first episode "Pilot"), episode titles of The Big Bang Theory always start with "The" and resemble the name of a scientific principle, theory or experiment, whimsically referencing a plot point or quirk in that episode.
It’s no secret that the cast of The Big Bang Theory gets paid out for every episode. But, who is the richest Big Bang Theory cast member? As it turns out, the show’s lead, Jim Parsons is the highest earning actor on the hit television series. The actor, who plays Sheldon Cooper, is worth $70 million and brings in $900,000 per episode on The Big Bang Theory (he and some of his castmates made $1 million per episode, but took a $100,000 pay cut so some of the other supporting roles could get a raise).

In the apartment building where Sheldon, Leonard and Penny (and later Amy) live, the elevator has been out of order throughout most of the series, forcing characters to have to use the stairs. Stairway conversations between characters occur in almost every episode, often serving as a transition between longer scenes. The Season 3 episode, "The Staircase Implementation" reveals that the elevator was broken when Leonard was experimenting with rocket fuel.[89] In the penultimate episode of the series, the elevator is returned to an operational state, causing Sheldon some angst.
While opening their wedding gifts, Sheldon and Amy cannot figure out the purpose of the glass shard Leonard and Penny gave them. They decide it must be a clue to a scavenger hunt to the real gift. They eventually end up in the coffee shop where they first met and find a locket in the lost and found that they think must be the real gift. The glass shard was actually a crystal chakra wand that Raj gave as a gift to Howard and Bernadette, who then re-gifted it to Leonard and Penny, who finally passed it on as a joke. Leonard and Penny find it less funny when they are given a coded thank-you note they have to use to unlock their WiFi. After making out with his employee Denise at Sheldon and Amy's wedding reception, Stuart finally asks Denise out on a date; while preparing for the date, he has a spray-tan accident, but she just finds it funny and still goes out with him. Raj is depressed everyone else has a relationship and asks his father to arrange a marriage.
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).

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. 
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