Watch The Big Bang Theory Online - Full Episodes

In the season finale while Penny worries that dating Leonard has ruined her for normal guys, Wolowitz and Koothrappali search for Sheldon's perfect match online.

Leonard recounts for Penny the story of how he met and moved in with Sheldon.

Leonard and Sheldon vie to catch the eye of a visiting female physicist.

When a fight leads to Penny and Leonard no longer speaking to each other, Sheldon goes to extraordinary lengths to try to keep them both happy.

Sheldon gets to square off on the bowling alley against nemesis Wil Wheaton, while Leonard and Penny have their own fight.

Sheldon's fear of speaking in public threatens his ability to accept an award.

The discovery of a prop ring from The Lord of the Rings movies puts a strain on the guys' friendship.

An appearance in traffic court jeopardizes Sheldon's chance to meet Stan Lee.

Leonard can only bring one guest on a trip to see CERN's Large Hadron Collider (the world's largest and highest energy particle accelerator) in Switzerland on Valentine's Day.

Sheldon ends up working with Penny at The Cheesecake Factory to help solve a physics problem.

A robbery of their apartment sees Leonard and Sheldon call on their friends to devise a security system.

Leonard is aghast to find out a secret of Penny's: she believes in psychics. Meanwhile, Sheldon tries to assist Koothrappali in picking up a girl at a university party.

Leonard fears the worst for his relationship with Penny when his mother visits, while Sheldon is nothing but thrilled with Beverly's presence.

Penny seeks Sheldon's help to better comprehend Leonard's physics work. Wolowitz sees green when Leonard and Bernadette become pals.

Barry Kripke embarrasses Sheldon on NPR, and the gang rallies to help him get his revenge. Wolowitz, desperate to keep his romance with Bernadette alive, gets advice from a fantasy version of Katee Sackhoff.

With the rest of the gang off on a desert camping trip, Sheldon must play hero to an injured Penny.

After a fight between Leonard and Penny has some influence on his life, Sheldon is forced to play peacemaker in order to get the couple back together.

Leonard gets tutored in understanding football by Sheldon, while a kite mishap puts a rift between Wolowitz and Raj.

After overhearing that Wil Wheaton is always entering a trading-card competition, Sheldon decides to enter it himself. Meanwhile, Penny sets up one of her friends on a date with Wolowitz.

Facing a return to India, Raj needs a job desperately enough to consider working for Sheldon. Meanwhile, Howard becomes a third wheel to Leonard and Penny.

Howard takes Raj along to check out the Goth dating scene. Sheldon uses chocolate as part of a behavior modification experiment on Penny.

While Leonard and Penny have problems trying to recover from an embarrassing hook-up, Sheldon and Howard bet their very best comic books trying to decide the species of a cricket.

Discovering that Leonard, Raj, and Howard fudged the results of his Arctic research expedition, a disgraced Sheldon heads home to Texas.

Leonard packs his bags for a three-month long science trip, and he and Penny talk about their feelings for each other in the second-season finale.

Stuart goes on a date with Penny, but during the date, she discovers something unexpected about Leonard. Meanwhile, Wolowitz needs help from his friends when he discovers a design flaw in his NASA project.

Leonard and Koothrappali escort a heartbroken Wolowitz to Las Vegas.

A fellow comic book enthusiast's hitting on Penny leads to Leonard and Howard going to a bar's Ladies' Night.

Penny squares off with a new beautiful female neighbor who may become the building's "new Penny."

Penny's business selling hair products out of her apartment gets out of hand when an over-caffeinated Sheldon takes charge.

Leonard, Sheldon, Wolowitz and Raj take a train trip to San Francisco and can hardly believe beautiful sci-fi actress Summer Glau is onboard.

A paintball game sparks a fight between Penny and Sheldon, and leads to romance for Wolowitz and Leslie.

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Sheldon helps Penny with a financial crisis, but his solution leads to a confrontation between Leonard and Penny's burly ex-boyfriend Kurt (Brian Patrick Wade).

The boys discover a conundrum which is baffling: The very source of Sheldon's social awkwardness seems to be his intelligence, is also the reason according to his new scientific procedure for making friends.

Penny's criticism of Wolowitz's romantic life pushes him into depression and AWOL for a fighting robot battle the rest of guys are depending on him for.

Sheldon irks the gangs with his obession with proper Christmas gift protocol. Lenoard's crushed when Penny dates a colleague of his.

Leonard ask for advice from Penny when his relationship with Stephanie begins moving too fast for him.

Sheldon becomes jealous when Leonard begins dating a woman Sheldon thinks is meant for himself.

A girl Wolowitz has a major crush on falls for Leonard.

When Sheldon bans Penny from the apartment for numerous minor infractions, she decides to retaliate.

Sheldon, as well as the others, are baffled when a young grad student begins to fall for him.

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Raj gets a swelled head and goes after Leonard's new crush, Penny, after being made famous in a magazine article leading to a disastrous result.

Penny is introduced to online gaming by Sheldon, and she quickly becomes addicted.

Leonard begins dating Sheldon's nemesis, Leslie Winkle, after seeing Penny with a handsome new guy.

Penny and Leonard go on a date, but when their night out goes awry, Penny finds a friend in Sheldon.

Leonard and Penny each seek a reluctant adviser, Sheldon, for a bit of wisdom concerning their date with each other.

Penny throws Leonard a surprise party after discovering that he doesn't celebrate birthday's, but becomes sidetracked by Sheldon, who, while at an electronics store, gets to live out one of his greatest fantasies.

The guys fight for Sheldon's twin sister when she arrives for a visit, but Leonard has a plan to get rid his rivals. Sheldon becomes her protector and decides he will pick out who she will go out with, despite her disapproval.

Leonard and the gang buy a time machine prop that was used in a classic 1960s movie. However, their thrill over having a replica time machine is shot down when Penny tells them that their obsession with comics and toys is "pathetic".

Sheldon finds himself out of the gang's Physics Bowl team and replaced with Leslie. Not taking this sitting down, Sheldon decides to start a team of his own to prove that he's the best out of the gang.

Leonard and Sheldon meet a 15-year-old physics prodigy and Sheldon is thrown off that the boy is younger and smarter than himself. This leads the boys to try to come up with a way to restore Sheldon's sense of self and ultimately decide to try to throw Dennis' research.

Penny nurses a sick Sheldon back to health, unaware that the guys refuse to do so because he makes such a lousy patient.

Leonard tells Penny a lie so that he doesn't have to hear her sing again, while Sheldon widens the lie, which makes Leonard in need to produce a phony cousin.

Sheldon and Leonard are asked to present their work at a physics conference together, but a fight ensues when Sheldon refuses to share the honor. When Penny steps in to mediate, things only get worse.

Koothrappali is overwhelmed after his parents arrange a blind date, but he discovers he actually can talk to the opposite sex after a few drinks; however, complications set in when his beautiful date is more interested in Sheldon.

Wolowitz seduces a friend of Penny's. As they have taken over Penny's apartment, Penny is forced to sleep on Leonard and Sheldon's couch. The guys are also short one member on their HALO team, and they ask Penny to join them.

When the guys are invited to Penny's Halloween party, Leonard sees it as an opportunity to get to know her friends and thereby get closer to her, but when her ex-boyfriend arrives, those plans take a turn for the worse. However, the evening isn't without its positive aspects for Leonard. Or for Rajesh.

Leslie invites Leonard to play cello in a musical ensemble, but they have to practice at the guys' apartment. Once practice is over, Leslie quickly seduces Leonard for a sexual encounter. Having absolutely no experience around anyone who has actually had sex, Sheldon seeks wisdom from Penny. In spite of initial trepidation, Sheldon decides that Penny's restaurant has a superior hamburger and will be a regular stop for him.

When Sheldon is fired, he tries to explore life outside the university. He goes food shopping with Penny, who is less than enthusiastic, and then conducts several scientific experiements in his apartment. When Leonard goes as far as buying a loom and weaving ponchos, Leonard decides to call Sheldon's mother for help.

Leonard is crushed when he sees Penny kissing another man in the hallway. Faced with the bleak reality of his chances with Penny, he turns his affections toward Leslie, a fellow scientist at the university. His advances, while partially successful, are less than satisfying. While Sheldon sees no possible future for Leonard and Penny, he feels sympathy for Leonard and suggests that he ask Penny on an actual date.

Leonard agrees to sign for a package for Penny, but when it turns out to be a massive unassembled entertainment unit, he and Sheldon must muscle the box up several flights of stairs - a task for which they are ill-equipped. Once in Penny's apartment, Sheldon is appalled at Penny's housekeeping practices and takes it upon himself to clean up somewhat until Penny gets home. That night, the "fuss budget" in him can't stand it anymore, so he sneaks back to finish the job (while Penny is asleep). Against his better judgement, Leonard helps. When Penny wakes to find that the guys were in her place, she feels violated and is furious, setting back Leonard's plans for romantic bliss.

Sheldon and Leonard are two brilliant, but socially inept theoretical physicists who share an apartment. In the series premier, they meet their new neighbor, Penny, a stereotypical beautiful dumb blond. In spite of the obvious hopelessness of the situation, Leonard is utterly smitten.