The two main characters (Sheldon Cooper and Leonard Hofstadter) are named after Sheldon Leonard.
Penny was an add-on from a different sitcom concept. She had been originally envisioned as more cynical and a hard drinker.
Raj's inability to talk to women except when drunk is based on an old coworker of executive producer Bill Prady when he worked at a computer company.
Sara Gilbert (Leslie) who plays a former girlfriend of Leonard's, also played Johnny Galecki's girlfriend in "Roseanne" (1988). Laurie Metcalf, who plays Sheldon's mother, was also his co-star in Roseanne.
Sheldon occasionally wears t-shirts featuring a specific color with a corresponding symbol. These shirts are from the "Green Lantern" comic book series, with each color representing a different corps of heroes or villains (e.g. Red Lanterns, Orange Lanterns, etc). In the comics, each color also represents a specific emotion: Red=Anger, Orange=Greed, Yellow=Fear, Green=Willpower/Courage, Blue=Hope, Indigo=Compassion, and Violet=Love. Not surprisingly, the color of shirt Sheldon wears in each scene is often a clue to his mood.
Some of the signs in the laundry room reads "No open containers on the machines", "Do not leave laundry unattended" and "Please keep your clothes on while doing laundry".
While Wil Wheaton was appearing on the show, he took a photo of the set's coffee table and posted it on his much-followed blog. Among the materials on the coffee table were: copies of "Mental Floss" magazine; The Sandman Vol. 6: Fables and Reflections by Neil Gaiman; a package of Pepperidge Farm Sausalito milk chocolate macadamia nut chunk cookies; a package of Redvines licorice; and a package of Tim's brand Original Flavor lightly salted potato chips.
Long before Kaley Cuoco joined the CBS sitcom as Leonard and Sheldon’s ditzy neighbor Penny, Chuck Lorre shot a pilot with unknown Canadian actress/VJ Amanda Walsh as his female lead. Walsh and her character — a street smart gal with a sensitive core — were both scrapped by Lorre after CBS rejected the pilot.
The writers have actual physicists that work on the show to be sure that everything the characters say (in relation to physics) are accurate.
The theme song for The Big Bang Theory is written and performed by The Barenaked Ladies.
The actors who play Leonard and Penny went out in real life but later broke up but remain friends.
Sheldon has two restraining orders against him.
Leonard Nemoy- Reason for the order is unknown
Stan Lee- Sheldon came into Stan Lee's house uninvited
Sheldon, Howard, and Leonard often make fun of Rahj and his hindu beliefs.
The judge in Sheldon's court case in the episode where Stan Lee appears is named J. Kirby, which references Jack Kirby who worked with Stan Lee to create many of Stan's early creations.
Ep. 1 Pilot We learn that Penny is from Omaha, Nebraska.
Ep. 1 Pilot Leonard is lactose intolerant and can't process corn.
Ep. 1 Pilot Koothrappali wears a hat with the number "42" on it. According to Douglas Adams’ The Hitch Hiker's Guide to the Galaxy book series, 42 is the answer to the ultimate question of life, the universe, and everything.
Ep. 2 The Big Bran Hypothesis After viewing Penny's messy apartment, Sheldon exclaims, "Great Caesar's ghost, look at this place," borrowing the exclamation that the editor of the Daily Planet, Perry White, often uses in Superman media.
Ep. 2 The Big Bran Hypothesis We learn that Howard is allergic to peanuts, and Sheldon is allergic to bees.
Ep. 3 The Fuzzy Boots Corollary Azeroth is the main world where World of Warcraft and other Warcraft games are based. The race Night Elf and the class Rogue are possible choices for characters.
Ep. 4 The Luminous Fish Effect We learn Sheldon has two siblings. We were already told of his twin sister who hostesses at Fuddrucker's.
Ep. 4 The Luminous Fish Effect Laurie Metcalf (Mary) is the second of Johnny Galecki's former Roseanne co-stars to guest-star on this show; the first being Sara Gilbert.
Ep. 5 The Hamburger Postulate When Leslie is freezing the banana with liquid nitrogen, she says that it is "320° below zero," which would be in Fahrenheit. As a scientist, she should have used the Celsius or Kelvin scale.
Ep. 5 The Hamburger Postulate Sheldon went to college when he was eleven.
Ep. 6 The Middle Earth Paradigm Leonard's middle name is Leakey.
Ep. 6 The Middle Earth Paradigm Howard reveals he has transient idiopathic arrhythmia.
Ep. 6 The Middle Earth Paradigm Howard mentions there being a "great show on VH1 about how to pick up girls," referring to their reality series, The Pick Up Artist.
Ep. 6 The Middle Earth Paradigm When Sheldon dressed up as the Flash, it looks like he was going for the Wally West form of the Flash.
Ep. 7 The Dumpling Paradox Responding to a knock at the door, Sheldon queries, "Oh, what fresh hell is this?" a phrase author and poet Dorothy Parker would reportedly answer her phone with.
Ep. 8 The Grasshopper Experiment In this episode, Sheldon claims that he is an Honorary Member of the Justice League.
Ep. 8 The Grasshopper Experiment The Koothrappalis revealed that Raj is 26 years old.
Ep. 9 The Cooper-Hofstadter Polarization The imagery of the guys beating their chests plays is an homage to 2001: A Space Odyssey.
Ep. 9 The Cooper-Hofstadter Polarization In this episode Leonard reveals he got his doctorate at age 24, while Sheldon got his at age 16.
Ep. 9 The Cooper-Hofstadter Polarization Leonard only has one tie, the reversible one, but in "The Hamburger Postulate", a different tie hangs on Leonard's door knob with a completely different pattern.
Ep. 9 The Cooper-Hofstadter Polarization The video of Sheldon and Leonard fighting that the guys watch on YouTube is different than what actually happened. When it actually happened Sheldon said "heads up, you people in the front row, this is a splash zone." On the video, Sheldon says "you people in the front row, heads up, this is a splash zone." Also on the video, right after Sheldon's line Raj looks at Howard's camera even though when it actually happened he clearly did not look at the camera.
Ep. 9 The Cooper-Hofstadter Polarization The "little misunderstanding" Sheldon refers to the Pope and Galileo having involves Galileo championing Copernicus' theory that the earth circled the sun, in opposition to the Catholic Church's official teaching that the earth was the center of the universe.
Ep. 9 The Cooper-Hofstadter Polarization When Penny asks Sheldon how he feels and he responds, "I don't understand the question," is a play on Spock's interplay with a Vulcan examination computer and his mother in Star Trek IV: The Voyage Home.
Ep. 10 The Loobenfeld Decay When Sheldon puts the chicken in the trash it goes in sideways with the plastic handles to the side. However, when he removes the lid, the chicken appears to be sitting upright with the handles on top for him to grab.
Ep. 10 The Loobenfeld Decay Howard snubs Leonard's apology over the fake symposium with, "No it's okay. It's your Millenium Falcon...you and Chewbacca do whatever you want in it. Me and Princess Leia here will find another way to spend the evening," all referencing Star Wars.
Ep. 10 The Loobenfeld Decay While discussing the Terminator universe with Leonard, Sheldon wonders why there'd be a need for a "petite, hot 17-year-old killer robot", referring to a character in Terminator: The Sarah Connor Chronicles.
Ep. 11 The Pancake Batter Anomaly The 3D chess that Sheldon and Leonard play is from a game played on the original Star Trek.
Ep. 11 The Pancake Batter Anomaly Howard's age is revealed as 26 years old.
Ep. 12 The Jerusalem Duality Sheldon says of his reproduction of Jerusalem that it'll be, "like that baseball movie, 'build it and they will come'," referring to the film Field of Dreams.
Ep. 12 The Jerusalem Duality Informed that Dennis has agreed to join the university, Sheldon exclaims, "the oracle told us little Neo was the one," taken from The Matrix.
Ep. 12 The Jerusalem Duality Howard holds a Masters degree in engineering.
Ep. 12 The Jerusalem Duality We learn Sheldon's middle name is Lee, according to his Stevenson award.
Ep. 13 The Bat Jar Conjecture Raj asks, "How come in Star Trek everyone's private parts are the same? No alien lady ever told Captain Kirk, 'Hey, get your thing out my nose,'." forgetting the prison fight scene that ends with the line, "not everyone keeps their genitals in the same place, Captain," in Star Trek VI: The Undiscovered Country.
Ep. 13 The Bat Jar Conjecture When the Perpetual Motion Squad is declared the winner and "We Are the Champions" by Queen begins to play, Wolowitz kneels on the floor and tears off his shirt just as Freddie Mercury used to do in concert.
Ep. 13 The Bat Jar Conjecture After having kicked Sheldon off their quiz team, the guys discuss replacements. Raj mentions TV's Blossom. The actress Mayim Bialik that played Blossom, regularly portrays Amy Farrah Fowler from the end of season 3 onwards.
Ep. 14 The Nerdvana Annihilation As Leonard attempts to take his memorabilia away to sell, Sheldon grabs a toy sword, wields it, and blocks the stairway, declaring "None shall pass!", imitating the Black Knight from Monty Python and the Holy Grail.
Ep. 15 The Pork Chop Indeterminacy When Raj asks Missy "...if you wonder, wonder, who wrote the book of love," he is borrowing lyrics from the 1957 song, "Book of Love", by the Monotones.
Ep. 16 The Peanut Reaction Penny asks Leonard if he'll be coming in to The Cheesecake Factory for his free birthday slice of cheesecake (although he's lactose intolerant). Actually, they only offer a free scoop of ice cream.
Ep. 17 The Tangerine Factor Sheldon asks Howard to help him learn to speak Chinese, though in "The Dumpling Paradox" Chen inquires about Howard by asking "Where is your annoying little friend who thinks he speaks Mandarin?"
Ep. 1 The Bad Fish Paradigm Sheldon mentions that his pre-packed emergency bag is, "recommended by the Department of Homeland Security...and Sarah Connor."
Ep. 1 The Bad Fish Paradigm Even though the opening presentation is the same, they changed the last shot (where the gang is eating) for an updated one.
Ep. 2 The Codpiece Topology The equation on the board behind the sofa says 3 x 3 x 3 = 8 x 1 10 which is incorrect.
Ep. 2 The Codpiece Topology Sara Gilbert is elevated to the main cast, but is only credited when she appears in an episode.
Ep. 3 The Barbarian Sublimation Wolowitz exclaims, "What the frack?," borrowing the word used by humans on the new Battlestar Galactica.
Ep. 3 The Barbarian Sublimation It is revealed that Penny is 22 years old.
Ep. 4 The Griffin Equivalency Sheldon tries to smile but only manages an odd grimace. Leonard mentions that they are, "not out to kill Batman," comparing his smile to The Joker from Batman.
Ep. 5 The Euclid Alternative Sheldon suggests a blue car in the driving simulator looks like the light blue of Luke Skywalker's light saber before the digitally reworked version, referring to the original Star Wars trilogy.
Ep. 6 The Cooper-Nowitzki Theorem Sheldon tries to invoke the Body Snatchers Clause in his friendship with Leonard, where Leonard agrees to help if pods are creating duplicates of people during their sleep. (Invasion of the Body Snatchers)
Ep. 6 The Cooper-Nowitzki Theorem Sheldon tries to invoke the Skynet Clause in his friendship with Leonard, where Leonard agrees to help if Sheldon creates an artificial intelligence that becomes self-aware and threatens the human race. (Terminator)
Ep. 7 The Panty Piñata Polarization The two models featured at the end of this episode, Analeigh Tipton and Samantha Potter, were both contestants and finalists in season 11 of America's Next Top Model.
Ep. 7 The Panty Piñata Polarization When Howard chants "One of us, one of us" he is referencing the film, Freaks.
Ep. 8 The Lizard-Spock Expansion Sara Rue also worked with Simon Helberg in the episode "Coup" of Popular and in the episode "It Takes A Pillage" of Less Than Perfect.
Ep. 9 The White Asparagus Triangulation The movie premise that Penny mocks Sheldon with is The Lake House.
Ep. 9 The White Asparagus Triangulation We learn Leonard uses "Kal-El" for all his passwords, a reference to the birthname of Superman.
Ep. 10 The Vartabedian Conundrum Sheldon got a stethoscope and a blood pressure cuff for his 12th birthday from his aunt Marion because, "she thought if I failed at theoretical physics that I should have a trade to fall back on."
Ep. 12 The Killer Robot Instability When Leonard gets off the phone with Howard, it is obvious that he wasn't making an actual phone call...he was in the contacts menu.
Ep. 13 The Friendship Algorithm A "raccoon with an distended scrotum" (as Wolowitz drew on the questionnaire) is known in Japan as a tanuki, a mythical mischevious raccoon-like creature with shapeshifting abilities and a representation of good luck.
Ep. 15 The Maternal Capacitance Leonard has a sister and a younger brother (named Michael), both more successful in their fields than he.
Ep. 16 The Cushion Saturation The line, "God, I love the smell of paintballs in the morning," is a play on the line in the 1979 film, Apocalypse Now, where Robert Duvall's character says, "I love the smell of napalm in the morning."
Ep. 16 The Cushion Saturation This episode marks the final appearance by Sara Gilbert as a main star. Signed to a contract as a series regular at season's start, writers were unable to find storylines for her, so they dropped her from the contract.
Ep. 17 The Terminator Decoupling As a fan of the show's often physics-centric humor, Nobel Laureate Dr. George Smoot contacted the show and was subsequently found a role as himself in this episode.
Ep. 17 The Terminator Decoupling The 'sci-fi female appearing as herself' role was originally offered to Battlestar Galactica's Katee Sackhoff, but she turned it down. Summer Glau took the role, instead.
Ep. 19 The Dead Hooker Juxtaposition Sheldon's repetition of "The horror" is taken from Joseph Conrad's novella, Heart of Darkness.
Ep. 21 The Vegas Renormalization It's revealed that Raj's middle name is Ramayan.
Ep. 22 The Classified Materials Turbulence A few weeks after this episode aired, the same problem with the toilets for the International Space Station that Howard tried to fix happened on the real Space Station.
Ep. 3 The Gothowitz Deviation Leonard informs Penny that they're watching Oshikuru: Demon Samurai. This is a fictional anime that first was referenced in Chuck Lorre's Two and a Half Men. In "The Salmon Under My Sweater", Charlie Harper was hired to compose the theme song when the manga was being adapted for television.
Ep. 5 The Creepy Candy Coating Corollary Sheldon mentions that Joel Schumacher is on his enemies list because he almost destroyed the Batman franchise, referring to the films Batman and Robin and Batman Forever which both got very bad reviews.
Ep. 7 The Guitarist Amplification Raj comments, "It's like my girl Beyoncé says: 'If you liked it, then you should've put a ring on it,'" quoting the chorus of her 2008 song "Single Ladies."
Ep. 8 The Adhesive Duck Deficiency In the final "Soft Kitty" scene, Penny references the film WALL·E with her quote, "You're so full of love, and you can save a plant and get fat people out of the floaty chairs."
Ep. 9 The Vengeance Formulation Katee Sackhoff turned down the role that went to Summer Glau for "The Terminator Decoupling", but eventually accepted a guest spot when Chuck Lorre approached her with the concept of her appearing as Howard's fantasy.
Ep. 11 The Maternal Congruence Beverly said she and her husband hadn't had sex in eight years. However, in "The Maternal Capacitance", she stated that she and her husband only had sex to have children and that Leonard is her youngest child.
Ep. 13 The Bozeman Reaction It is revealed that Sheldon and Leonard possessed a TV, two laptops, four external hard drives, a PS2, a PS3, an Xbox, an Xbox 360, a classic Nintendo, a Nintendo, a SuperNintendo, a Nintendo 64, and a Wii before they were robbed.
Ep. 14 The Einstein Approximation The original script for this episode only had the idea of Penny slipping on marbles. It was up to the show's scientists to figure out what Sheldon would be doing to have marbles on the ground.
Ep. 16 The Excelsior Acquisition We learn Leonard uses the 1967 Spider-Man theme as a ringtone.
Ep. 19 The Wheaton Recurrence After bringing Penny food to calm her and explaining he learned it from the comic strip Cathy, Sheldon tells her, "if you were a cat, I would have brought you a lasagna," referring to the title cat in Garfield, whose favorite food is lasagna.
Ep. 22 The Staircase Implementation In Howard's room of 2003 they showed a Roboraptor and a slimline PS2, neither of which was available at that time.
Ep. 23 The Lunar Excitation Before firing the laser, Sheldon says "Make it so," borrowing Jean-Luc Picard's typical verbage on Star Trek: The Next Generation.
Ep. 5 The Desperation Emanation It is revealed that Raj has a girlfriend and since she is deaf it does not matter that he cannot talk to her since she cannot hear him.
Ep. 18 The Prestidigitation Approximation At the end of this episode, it becomes clear that everyonr was pretending that Howard's trick worked to piss off Sheldon.