Leonard Leakey Hofstadter[2][3] is a fictional character portrayed by Johnny Galecki and one of the main protagonists in the 2007-2019 CBS sitcom, The Big Bang Theory.

Penny is Leonard's next-door neighbor (across the hall) and main love interest, and the teasing of romance between the two of them is a major force driving the series.

Although Leonard feels at home with his geek colleagues, he wants to be more social; out of the four main male characters, he has the least difficulty interacting with "non-geek" individuals.

While certainly a geek by most definitions, he is far less socially inept than Sheldon (who is unable to grasp social norms and is often even dismissive of them), Raj (who was unable to talk to women unless he was under the influence of alcohol or anti-anxiety prescription drugs, and even after getting over his handicap still tended to speak inappropriately), and Howard (who was often seen as "creepy" when flirting with women, including Penny).

As seen in season 1 episode "The Hamburger Postulate", he and Leslie Winkle have sex, after meeting to practice their instruments, which Leonard calls "a little musical foreplay".

These include deliberately triggering Sheldon's obsessive compulsive problems, mocking Howard and Raj whenever they slip up, and making sniping comments when Penny's commitment issues flare up.

In order to avoid detection by a sick Sheldon and Penny, who was nursing him, Howard and Raj helped him to navigate through the living room by using a helmet camera.

He also suffers from asthma, as it is apparent in multiple episodes that he needs his inhaler in order to perform a variety of activities including sports and sex.

During his relationship with Leslie Winkle, he tells her that many members of his family have died from heart disease and thus he has a genetic disposition to the condition.

All of the members of Leonard's family are accomplished scientists, except for his younger brother Michael, who is a tenured law professor at Harvard University.

She has a personality almost identical to Sheldon's, including strict speech patterns, lack of social conventions, and attention to detail, and she is principally responsible for Leonard's difficult childhood.

Due to his mother's views, Leonard never celebrated his birthday or Christmas when he was a child,[14] and when she visited him she was not impressed with his unoriginal research.

Leonard revealed to Penny that he built a "hugging machine" when he was young in order to compensate for the lack of affection from his mother.

Leonard also dislikes his memories of Christmas, not just because he never celebrated it, but also because instead of gifts the Hofstadter kids had to turn in papers to "Santa" which were graded the next day.

However, she reveals that she is doing research for her new book that highlights her parenting technique although difficult most of the time, directly resulting in the success of her children.

However, they seem to have developed a good, affectionate rapport by the time of Leonard and Penny's impending second wedding ceremony in the season 9 finale.

[22] Although Leonard's attempts to disprove the existence of dark matter were made moot by the work of another physicist, Leonard helped to solidify proof of the hypothetical matter's existence with a photomultiplier, and was invited as keynote speaker to a topical conference by the Institute of Experimental Physics for his successful research on super solids.

[23] From the season 6 finale, "The Bon Voyage Reaction" Leonard heads to the United Kingdom to work on a physics project seeking the hydrodynamics equivalent of the Unruh effect, sponsored by Professor Stephen Hawking, for three months on a ship in the North Sea.

In season 8, Leonard has a revelation about Superfluid Vacuum and with Sheldon's help was able to come up with a paper that impresses the scientific community including Stephen Hawking.

This theory was later used as a foundation for a Quantum Guidance System that Sheldon, Leonard and Howard develops for the US Air Force in Season 10.

While Sheldon and Raj stay to help him try to fix the problem, Howard asks Leonard to take Stephanie home, where they start making out in his car.

[32] However, the relationship ended after eight months,[33] after Leonard told Penny he loved her and took umbrage at her inability to reciprocate, which led to friction that was exploited by guest star Wil Wheaton who appeared as a fictionalized version of himself, who observed and exacerbated it in order to break them up in the middle of a bowling match Wheaton's team was having against Leonard, Penny, Sheldon, Howard and Raj.

In the third-season finale, "The Lunar Excitation", after Penny's failed attempts to move on from her relationship with Leonard, she becomes intoxicated and has sex with him.

Raj strongly opposes this, and Penny is secretly uncomfortable with Leonard dating someone else, eventually breaking down and crying while talking with her friend Amy Farrah Fowler.

When they are walking up the stairs to Leonard's apartment, he says he pulled something in his crotch doing the Hokey Pokey, and thanks to Amy for breaking the head off the ice swan sculpture to help him.

In the next episode, Leonard tells Sheldon that he is single, implying that he and Priya have broken up; his actions since then have confirmed their split occurred.

During their renewed relationship Penny has dismissed comments about him ever leaving or dumping her or about worrying about his unfaithfulness around other women and strippers as in the episode "The Stag Convergence".

In the "Higgs Boson Observation", Sheldon hires grad student Alex Jensen to review his childhood journals for any potential ideas that might win him a Nobel Prize.

Penny becomes hurt by the revelation, and even though she claims to forgive Leonard, there is clearly tension between them, leaving the season ending in a cliffhanger as to whether or not they will go through with the wedding.

She barely remembers the incident and quickly grows bored with Leonard's ramblings, taking a sarcastic and dismissive tone toward him.