- Main article: List of characters from The Big Bang Theory
- Sheldon Cooper, Ph.D., portrayed by Jim Parsons, is a theoretical physicist. Originally from East Texas, he was a child prodigy, starting college at the age of 11, right after completing the fifth grade. He has an IQ of 187 and exhibits a strict adherence to routine; a lack of understanding of irony, sarcasm and humor; and a complete lack of humility.
- Leonard Hofstadter Ph.D., portrayed by Johnny Galecki, is an experimental physicist with an IQ of 173 who received his doctorate when he was 24 years old. He shares an apartment with colleague and friend Sheldon Cooper and is the straight man of the series.
- Penny, portrayed by Kaley Cuoco, is the attractive, blonde neighbor who lives across the hall from Sheldon and Leonard. She has aspirations of a career in show business, and has been to casting calls and auditions but has not been successful thus far. To pay the bills, she works as a waitress at The Cheesecake Factory. Her last name has never been revealed.
- Howard Wolowitz, M.Eng., protrayed by Simon Helberg, works as an aerospace engineer. He is Jewish, and lives with his mother. Unlike Sheldon, Leonard, and Raj, Howard lacks a Ph.D. He defends this by pointing out that he has a master's degree in Engineering from the Massachusetts Institute of Technology and the apparatus he designs are actually built and launched into space, unlike the purely abstract work of his friends. He provides outrageous pick-up lines and fancies himself a ladies man with suitably unimpressed reactions from Penny; however, he has limited success with other women.
- Rajesh Koothrappali, Ph.D., portrayed by Kunal Nayyar, originally comes from New Delhi, India. Known commnly as "Raj", he works as a particle astrophysicist at Caltech. He is very shy around women and is physically unable to talk to them unless he drinks alcohol. However, he has much better luck with women as opposed to his overly confident best friend Howard.
- Leslie Winkle, Ph.D., portrayed by Sara Gilbert, is an experimental physicist who has casual sex with both Howard and Leonard during the show's airing. She does not get along well with Sheldon and frequently mocks him.
Season two begins to deal with character development, including Sheldon becoming more and more obsessive. Leonard and Penny's relationship takes a turn for the worse when they break up, but is quickly resolved. Soon their relationship progresses into a stronger friendship than before, while Sheldon and Penny's friendship slowly begins. In the season finale Penny hints at her true feelings for Leonard. Howard's character remains similar to that of season one in his attempts to seduce women, managing to start a short-lived sexual relationship with Leslie Winkle in the second half of the series, while Rajesh manages to briefly apologize to Penny without having consumed alcohol.
|Episode (Season)||Title||Original Air Date||Directed By||Description|
|The Bad Fish Paradigm||September 22, 2008||Mark Cendrowski||Penny and Leonard return from their date but she gives him a brush off when he tries to get a second date. She confesses to Sheldon that it is because she thinks she isn’t smart enough to keep his interest.|
|19||2||The Codpiece Topology||September 29, 2008||Mark Cendrowski||Sheldon meets with Penny on the stairs while playing Mario. Penny is dating a new guy, so Leonard jumps at the chance to date Leslie Winkle again. Sheldon hates Leslie and resents being exiled to the staircase to play Mario alone. Eventually he confronts Leslie about her scientific beliefs.|
|20||3||The Barbarian Sublimation||October 6, 2008||Mark Cendrowski||Sheldon becomes distracted from playing Age of Conan by Penny, who has accidentally attempted to use her car keys to open her apartment door. Sheldon learns that Penny is frustrated by her failures in life.|
|21||4||The Griffin Equivalency||October 13, 2008||Mark Cendrowski||Raj discovers a trans-neptunian planetoid and gets featured in People Magazine as a result. The fame soon goes to his head and he annoys all his friends.|
|22||5||The Euclid Alternative||October 20, 2008||Mark Cendrowski||Leonard is working nights at the university and can’t drive Sheldon to work. Sheldon annoys all his possible chauffeurs and fails to learn to drive.|
|23||6||The Cooper-Nowitzki Theorem||November 3, 2008||Mark Cendrowski||A grad student, Ramona Nowitzki, begins pushing Sheldon to concentrate all his spare time on his scientific endeavours. The loss of his freedom and leisure time upsets Sheldon but he can’t stop her.|
|24||7||The Panty Piñata Polarization||November 10, 2008||Mark Cendrowski||Penny touches Sheldon's onion rings and is soon banished from his apartment. They both refuse to apologize and begin to escalate their petty behavior. Leonard eventually steps in to bring peace. Meanwhile Howard and Raj learn about America’s Next Top Model.|
|25||8||The Lizard-Spock Expansion||November 17, 2008||Mark Cendrowski||A smitten Wolowitz thinks he's found his soul mate, until the woman, Dr. Stephanie Barnett, meets Leonard. Wolowitz invites Stephanie to drive the Mars rover, which winds up stuck in a ditch.|
|26||9||The White Asparagus Triangulation||November 24, 2008||Mark Cendrowski||Sheldon is concerned that Stephanie is the only girl Leonard has dated that he "finds tolerable". He tries to force himself into conversations with "urban slang" to "help" Leonard in the relationship with Stephanie.|
|27||10||The Vartabedian Conundrum||December 8, 2008||Mark Cendrowski||Stephanie Barnett has moved in with Leonard and Sheldon, according to their roommate contract, though Leonard denies that she is living with them. She tries helping Sheldon with a ringing in his ear, but is unable to help him. Penny then proves to Leonard that he and Stephanie are living together.|
|28||11||The Bath Item Gift Hypothesis||December 15, 2008||Mark Cendrowski||Wizard physicist and all around hunk David Underhill starts working with Leonard. He soon asks Penny out much to Leonard’s distress. He confronts Penny about why she isn't with him and discovers Underhill was married. Meanwhile, Sheldon obsesses about getting Penny a Christmas present of equal value while Penny has a surprise for him.|
|29||12||The Killer Robot Instability||January 12, 2009||Mark Cendrowski||Howard builds a robot called MONTE (Mobile Omnidirectional Neutralization and Termination Eradicator) to enter into the Southern California Robot Fighting League Round Robin Invitational. When Penny tells Howard what she really thinks of his sleazy come-ons he is hurt and withdraws to his home.|
|30||13||The Friendship Algorithm||January 19, 2009||Mark Cendrowski||Socially awkward, Sheldon develops a scientific procedure (the titular "Friendship Algorithm") for making friends. He needs to use an open science grid computer that is in Barry Kripke's office, but Leonard tells Sheldon that only Barry's friends can use it.|
|31||14||The Financial Permeability||February 2, 2009||Mark Cendrowski||Penny is short on her rent money and so Sheldon lends her some of his spare cash. She feels guilty about not paying it back and still can’t pay her electricity bills.|
|32||15||The Maternal Capacitance||February 9, 2009||Mark Cendrowski||Leonard's mother (Dr. Beverly Hofstadter) comes to visit. She is a cold, analytical neuroscientist. She drives Penny and Leonard to drink with her psychoanalysis while Sheldon loves her. Leonard ruins his chance to sleep with Penny.|
|33||16||The Cushion Saturation||March 2, 2009||Mark Cendrowski||Penny accidentally shoots Sheldon's sofa cushion with a paint ball gun. He is deeply disturbed by this turn of events and is not happy with the cushion even once it has been dry cleaned.|
|34||17||The Terminator Decoupling||March 9, 2009||Mark Cendrowski||A train trip to a conference in San Francisco takes a major detour when Leonard, Sheldon, Wolowitz, and Raj discover that their favorite sci-fi actress, Summer Glau, is on board. But the fan boy frenzy quickly gives way to a heated mass debate when they realize one of them will have to approach her.|
|35||18||The Work Song Nanocluster||March 16, 2009||Peter Chakos||Penny starts her own business and asks Sheldon for help. The other guys become involved and they end up spending all night trying to ship an order for one thousand “Penny Blossoms”.|
|36||19||The Dead Hooker Juxtaposition||March 30, 2009||Mark Cendrowski||A new tenant, Alicia, moves in to the apartment above the guys. Sheldon's only concern is with the noise she might make, whereas the other guys are all too quick to help her out with all her needs. Penny becomes increasingly jealous of the way they treat her and the fact that Alicia is commanding their attention.|
|37||20||The Hofstadter Isotope||April 13, 2009||Mark Cendrowski||It’s "Anything-Can-Happen-Thursday" for the guys and they end up with Penny at the comic book store. She goes on a date with the owner Stuart and Leonard is jealous. He suggests that they all head to a bar to meet women.|
|38||21||The Vegas Renormalization||April 27, 2009||Mark Cendrowski||Leslie ends her friends with benefits relationship with Howard. Sheldon suggests the guys go to Vegas to help Howard get over the pain. They do and end up hiring a prostitute (played by Jodi Lyn O'Keefe) to give him the Jewish girlfriend experience.|
|39||22||The Classified Materials Turbulence||May 4, 2009||Mark Cendrowski||Howard has designed a space toilet for the International Space Station. He realises that it is going to malfunction and asks the guys to help him fix it. Stuart has another date with Penny and asks Leonard for advice. Leonard gives him bad advice and feels terrible about it.|
|40||23||The Monopolar Expedition||May 11, 2009||Mark Cendrowski||After Sheldon wakes up the president of Caltech in the middle of the night simply to ask a question, he is offered to spend three months doing research on the North Pole in an attempt to prove the validity of string theory. Leonard, Howard and Raj are overjoyed about him leaving until they join the expedition. Penny is not happy about Leonard going.|