Everything might change when Leonard offers Penny a spur of the moment invitation to a romantic dinner for two

Amy is upset with Sheldon when he isn't impressed with her recent accomplishment; Howard struggles with Bernadette's aversion to children.

Leonard confronts his fears when he's contacted by a bully who tormented him in high school

Sheldon becomes jealous when Stuart from the comic-book store asks Amy out on a date, and Sheldon's forced to reconsider his relationship with Amy

Leonard and Penny attempt to hang out alone; and Sheldon tries to overcome his fear of birds

Amy is crushed when Bernadette and Penny go shopping for wedding dresses without her

Leonard feels threatened when a charming comic-book artist comes between him and Priya; and Sheldon is determined to scare his friends on Halloween

Sheldon's mom visits, and he competes for her attention, which seems more focused on his friends.

Leonard and Sheldon attend a party at the home of Sheldon's nemesis, Wil Wheaton (in a cameo along with Brent Spiner).

Raj meets a woman with whom he's able to talk.

Leonard and Amy get to know each other better, while Howard and Bernadette spend the weekend with Mrs. Wolowitz.

Penny and Sheldon clash, and Amy is caught in the middle of their battle; and Leonard looks for ways to keep his long-distance romance with Priya alive.

In the fifth-season opener, Penny hits a low point when she realizes what she's done by sleeping with Raj; and Sheldon becomes the leader of the paintball team

In the fourth-season finale, Raj moves in with Sheldon when Leonard and Priya's "Star Trek" bedroom fantasy is too much for him; and the guys tease Howard when Bernadette gets her PhD, making Howard the only one without ...

Howard finally tells his mother that he's marrying Bernadette, and the news puts her in the hospital.

Raj makes an attempt to cure his phobia of talking to women; and Penny spies on Leonard and Priya with the help of Bernadette.

Priya uses her legal expertise to rip apart Sheldon's roommate agreement, and the women take the flustered Sheldon out dancing

Sheldon and Amy spread gossip as part of an experiment; and Howard and Bernadette's relationship gets serious. Physicist Brian Greene has a cameo

The guys become sleuths to find out who hacked Sheldon's online-game account

Leonard is forced to choose between Penny and his new girlfriend, Priya; Howard's magic trick drives Sheldon crazy.

Sheldon is upset when he realizes Leonard is the center of their group of friends.

Wolowitz considers taking things with Bernadette to the next level; Leonard and Raj's sister rekindle their relationship.

A wealthy donor makes Leonard question what he is willing to do for science.

Sheldon takes acting lessons from Penny in hope of becoming a better teacher; Koothrappali fantasizes about his friend's girlfriend.

Bernadette runs into her ex-boyfriend (Rick Fox) while attending a science event with Wolowitz.

Leonard's idea for a smartphone app derails his friendship with Sheldon.

The guys enter a costume contest with Penny's new boyfriend.

Amy realizes she has feelings for Penny's ex; Koothrappali and Wolowitz try to prove who would be a better superhero.

Penny asks Leonard to lie to her father (Keith Carradine); Howard, Raj and Bernadette pull an all-nighter at a telescope.

While the guys camp out for a long-awaited movie screening, Penny and Bernadette invite Amy to her first slumber party.

Howard baffles Sheldon when he won't accept his apology; and an FBI agent (Eliza Dushku) interviews the guys for Howard's security clearance.

Leonard keeps an affair secret from Sheldon, Howard and Raj.

Leonard realizes he's the only one without a girlfriend, while Sheldon wants to get rid of his.

An embarrassing secret of Wolowitz's is revealed; Raj and Sheldon get into a war at work.

When Sheldon searches for an alternative to human companionship, the guys become concerned.

Realizing he will not live long enough to download his consciousness into a robot body, Sheldon tries to extend his life span.

Penny tags along on Sheldon's first date; Wolowitz finds a new use for a robotic arm.

While Penny worries that dating Leonard has ruined her for normal guys, Wolowitz and Koothrappali search for Sheldon's perfect match online.

Leonard tells Penny about how he met Sheldon for the first time...and what happened to the elevator.

Leonard and Sheldon compete for the attention of a famous female physicist.

When Leonard and Penny aren't speaking, Sheldon goes to extremes to keep them both happy.

A fight between Leonard and Penny threatens their relationship, while Sheldon battles Wil Wheaton in bowling.

Sheldon's friends come to his aid when his fear of public speaking stands between him and a coveted award.

When the guys find a ring from "The Lord of the Rings" at a garage sale, it threatens to tear them apart.

Sheldon winds up in traffic court when he gets a ticket because of Penny.

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's search for the answer to a physics problem leads him to work at the Cheesecake Factory with Penny.

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's mother (Christine Baranski) visits, which pleases Sheldon but upsets Leonard. .

Sheldon attempts to help Penny understand Leonard's work, and Wolowitz becomes jealous when Leonard starts hanging out with his new girlfriend.

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 Leonard, Howard and Raj away camping in the desert, an injured Penny has only Sheldon to rely on.

A squabble between Leonard and Penny sees Sheldon playing mediator, though out of his own incovenience from the sitiation rather than altruism.

Sheldon teaches Leonard how to understand football while a kite-fighting incident threatens Howard and Raj's friendship

Sheldon seeks revenge on his nemesis, Wil Wheaton; Wolowitz wants Penny to set him up with one of her friends.

Raj must find a new job or be sent back to India, but Sheldon has a solution, which leaves Leonard and Penny dealing with Howard as third wheel.

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

Sheldon and Howard wager comic books over a cricket's species. Meanwhile, a less-than-perfect romantic night agonizes Leonard and Penny.

Sheldon flees back to Texas in disgrace when he learns the guys tampered with his arctic expedition data, forcing the guys to follow him and threatening Leonard's hopes for a romantic encounter with Penny.

Leonard and Penny come to terms with their feelings for each other, as Leonard prepares himself for a three-month long science trip.

Things begin to look up for Leonard when Penny makes a surprising revelation on her second date with Stuart. Meanwhile, Wolowitz enlists his friends' help when he discovers a critical design flaw in his project for NASA....

Leonard and Koothrappali take a heartbroken Wolowitz to Las Vegas, leaving Sheldon locked out of his apartment and forced to bunk with Penny.

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

Penny feels threatened when another sexy woman moves into the apartment building.

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

The gang is thrilled when they learn Terminator: The Sarah Connor Chronicles' Summer Glau is also aboard their train to San Francisco.

Paintball leads to a squabble between Sheldon and Penny, as well as romance between Howard and Leslie.

Leonard suffers through a dreadful visit from his mother (Christine Baranski) that ends up bringing him and Penny closer.

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).

Sheldon creates a 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.

Penny informs Sheldon that she intends to give gifts to him and Leonard on Christmas which happens to be around the corner. After considerable debate with Penny, Sheldon agrees to also give a gift to Penny on Christmas.

Leonard's relationship with Stephanie is moving too fast for him, so he runs to Penny for advice.

Sheldon is overexuberant that Leonard finally has a girlfriend matching his high standards.

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, and his friends, are mystified when a younger grad student is attracted to him.

An intervention is used to try to force Sheldon to overcome his fear of driving.

Full of confidence following getting a paper published, Raj hits on Penny.

Sheldon creates a monster when he introduces Penny to online gaming.

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

Penny opens up to a reluctant Sheldon after a less-than-stellar first date with Leonard.

Leonard comforts Penny after she breaks up with her boyfriend, again. After asking her out on a date they each turn to Sheldon for advice. Also, Howard teaches Sheldon Mandarin so he can complain about his not-so-tanger...

After finding out Leonard has never had a birthday party Penny decides to throw one for him (and forces Sheldon to join her). Unfortunately things do not go as planned and Wolowitz ends up in the emergency room.

When Sheldon's attractive twin sister Missy comes to visit the guys fight over her. Sheldon ends up trying to be protective of her, much to her displeasure.

Leonard buys a replica time machine online. As the boys move it into the apartment they accidentally make Penny late for work causing her to lash out at Leonard for his nerdy ways. Upset, Leonard decides to get rid of al...

The guys decided to compete in a physics bowl but find Sheldon's showboating too much and kick him off the team. Upset, Sheldon forms his own team to compete against them.

Sheldon and Leonard look for a way to derail a 15-year-old physics prodigy's research.

When Sheldon gets sick, Leonard and the boys know what's in store and avoid him like the plague, leaving an unsuspecting Penny to nurse the world's worst patient back to health.

After hearing Penny sing, Leonard lies to avoid seeing her perform, but Sheldon takes it one step further with a more elaborate lie that leads to the appearance of his non-existent cousin.

When Sheldon and Leonard are invited to present their joint findings at a physics conference, Sheldon refuses. But when Penny steps in to mediate, it only deepens the rift between the two best friends.

Koothrappali's parents want him to go out with another Indian woman with the hopes of initiating a pre-arranged marriage. But his date turns out to have more in common with Sheldon.

When Wolowitz hooks up with Penny's old friend, it ruins "Halo Night" for the rest of the guys.

The guys are invited to Penny's Halloween costume party. During the party, Leonard has a run-in with Penny's ex-boyfriend.

Leonard and his colleague Leslie spend the night together. But Leslie tells Leonard after the fact that she's only interested in a one-night stand.

Sheldon's mother visits to get him out of the funk he's in after he is fired for disrespecting his new boss.

Leonard gets upset when he discovers that Penny is seeing a new guy, so he tries to trick her into going on a "date" with him.

Penny is furious and more than a little freaked out with Leonard and Sheldon when they sneak into her apartment overnight, and clean it while she is sleeping.

A pair of brilliant theoretical physicists meet a woman who shows them how little they know about the real world.