Leonard then decides to confess his behavior to priya via skype, but he is shocked to learn that priya has also betrayed him, sleeping with her former boyfriend; priya defends that they both slipped up a little, but leonard corrects her saying that he slipped a little, while she slipped a lot, bringing the conversation to a strained end.
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.
Penny attended, but did not graduate from, community college (she briefly goes back to school in season 6).
^ during season 5, episode 21 ("the hawking excitation"), raj states that sheldon had begged three times before: he begged the fox network not to cancel firefly, he begged tnt to cancel babylon 5, and when he got food poisoning, he begged a deity he does not believe in to end his life quickly.
One of leonard's former girlfriends who is mentioned on several occasions is joyce kim, who did not appear until the third season.
Leonard later goes to a wedding with amy, and they seem to take a liking to each other, though sheldon disapproves, karate-chopping leonard and saying, "she's not for you.
His various girlfriends have tried to change his outfits: penny had no success while stephanie got him to wear dress shirts and sweaters (which he found uncomfortable), and priya got him to briefly switch to dress shirts, waistcoats and dark slacks; he reverted to his original outfits after she left for india, but started eschewing his hoodies in favor of unbuttoned safari shirts and non-hooded jackets towards the end of the seventh season, a trend that continued into successive seasons.
In "the benefactor factor", an older rich woman propositions leonard in exchange for a large donation to the physics department; he initially resists and she admits that she planned to make the donation anyway, after which they engage in a one-night stand.
Leonard, concerned that he was trying to sabotage his relationship with penny, meets with the woman he kissed on the boat.
When did penny and leonard start dating in real life
Bialik as amy farrah fowler (guest star season 3, starring since mid-season 4) is a woman selected by an online dating site as sheldon's perfect mate after raj and howard secretly set up an account using sheldon's information, prompting raj to comment, "we finally have proof that aliens walk among us!
Consequently, sheldon slowly starts to open up over the rest of the season, and starts a more intimate relationship with amy.
Hit sitcom also featured their characters penny and leonard dating last year, but now both the real and fake relationship are over.
Leonard is one of four characters to appear in every episode of the series, along with sheldon, howard wolowitz and raj koothrappali.
In the first episode, sheldon and leonard discuss leonard's grandmother, who had visited them on thanksgiving the year before.
He has lived with sheldon for several years, leonard knows him better than his other colleagues: this has led sheldon to believe (slightly erroneously) that leonard is his best friend.
 in the second-season finale, when leonard, sheldon, howard, and raj were due to leave for a three-month scientific expedition to the north pole, penny privately expresses sadness, and wishes that he would not leave.
In addition, he is often shown to be quite jealous, often feeling insecure and defensive whenever penny interacts with another guy, and on one occasion trying to frighten off a british college student she was doing a project with, because penny commented positively on his accent.
Chuck lorre and cbs entertainment president nina tassler also credit the success to the influence of molaro, in particular the deepening exploration of the firmly established regular characters and their interpersonal relationships, such as the on-again, off-again relationship between leonard and penny.
When did leonard and penny date in real life
Leonard decides that since they are no longer dating, he can be honest and does not have to pay for everything or do whatever penny wants.
After another dispute with sheldon, leonard tries to move in with penny in "the spoiler alert segmentation", however she is not ready.
Amy also believes she and penny are best friends ("besties," as she puts it), a sentiment that penny does not initially share.
First row from left: rajesh koothrappali, leonard hofstadter, penny, sheldon cooper, and howard wolowitz, second row from left: bernadette rostenkowski-wolowitz and amy farrah fowler.
 until howard's marriage to bernadette in the fifth-season finale, howard's former living situation led leonard's psychiatrist mother to speculate that he may suffer from some type of pathology, and sheldon to refer to their relationship as oedipal.
However, despite his iq of 187, he finds many routine aspects of social situations and life in general difficult to grasp and navigate.
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.
During stuart's first appearance, the guys bring penny along to the store, and he manages to ask her on a date.
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.
 in season four, it was revealed that leonard has asthma as well, and also has to use an inhaler after intercourse.
In the season finale, leonard leaves for an overseas job for four months and penny is confident enough in their relationship to wait for him.
It is unclear whether or not leonard has noticed this, though he clearly is still interested in penny.
Various points in the show, sheldon also says that leonard gets carsick unless he sits in the front seat.
Leonard then tells her that when she is ready to get married, she can propose to him.
"physicist to the stars: real-life physicist/ucla professor david saltzberg's consulting relationship to the show".
In the episode "the ornithophobia diffusion", when there is a delay in watching star wars on blu-ray, howard complains, "if we don't start soon, george lucas is going to change it again" (referring to lucas' controversial alterations to the films) and in "the hot troll deviation", katee sackhoff of battlestar galactica appeared as howard's fantasy dream girl.
After a night out of bonding and drinking with penny in which she notices an attractive busboy, beverly kisses sheldon but realizes she "would rather have the busboy".
After penny suggests having sex in "the launch acceleration", leonard breaks the mood by proposing to her.
The geekiness and intellect of the four men are contrasted for comic effect with penny's social skills and common sense.
 subsequently, they break up because penny felt she was not as smart as leonard would like.
Is possible that leonard is based on the show's co-creator, bill prady, back when he was a computer programmer.
His mother, beverly (christine baranski), is cold and distant, a neuroscientist and psychiatrist who provided little, if any, nurturing when leonard was growing up.
They later meet and penny does have the courage to tell him "no" and not break up with him as she did two years previously when he told her that he loved her.
However, when leonard tells penny that he loves her, she realizes she cannot say it back.
Show is primarily 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 later becomes a pharmaceutical representative and 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.
Nevertheless, leonard often finds himself explaining sheldon's behavior or serving as a mediator between him and other people in many situations.
Leonard explains to penny that he moved in despite the difficulty of living with sheldon because the rent was so reasonable.
Big bang theory started off slowly in the ratings, failing to make the top 50 in its first season (ranking 68th), and ranking 40th in its second season.
 leslie briefly started dating leonard in season 2, but when he failed to back her up in an argument with sheldon, she dumped him.
The third season premiere, leonard and penny finally started a romantic relationship and had intercourse for the first time.
Leonard and penny decide to elope to las vegas in the season 8 finale, but beforehand, wanting no secrets, leonard admits to kissing another woman, mandy chow (melissa tang) while on an expedition on the north sea.
Leslie breaks up with leonard when he sides with sheldon in his support for string theory rather than loop quantum gravity.
" 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".
While certainly a geek by most definitions, he is 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).
For instance, when penny and sheldon engaged in a fierce dispute, leonard gave penny sheldon's mother's phone number so she would call her son and scold him for his behavior, thus ending the fight.
Kaley cuoco began being credited as kaley cuoco-sweeting starting with the season 7 episode the convention conundrum following her marriage to tennis player ryan sweeting.
When bernadette takes an interest in leonard's work, it makes both penny and howard envious and results in howard confronting leonard, and penny asking sheldon to teach her physics.
Afterwards, a heartbroken raj finally speaks to penny without having consumed alcohol and overcomes his selective mutism.
Time, supporting characters have been promoted to starring roles: leslie winkle, a physicist who dated leonard and howard; neuroscientist amy farrah fowler, who joins the group after being matched to sheldon on a dating website (and later becomes sheldon's girlfriend); bernadette rostenkowski, howard's wife (previously his girlfriend), a microbiologist and former part-time waitress alongside penny; stuart bloom, the cash-strapped owner of the comic book store the characters often visit; and emily sweeney, a dermatologist who dated raj.
Actors were first credited as guest stars and later promoted to main cast:Sara gilbert as leslie winkle (recurring season 1, starring season 2, guest star seasons 3 and 9) a physicist who works in the same lab as leonard.
In season 1, she dates mike (andrew walker), whom she later dumps after he posts details about their sex life on his blog.
To his mother's views, leonard never celebrated his birthday when he was a child, and when she visited him she was not impressed with his unoriginal research.
 leonard also dislikes christmas because instead of gifts the hofstadter kids had to turn in papers to "santa" which were graded the next day.
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.
However, he changed his mind when he saw that penny was already dating another man.