Leonard Leakey Hofstadter, Ph.D., is a fictional character on the CBS television series The Big Bang Theory, portrayed by actor Johnny Galecki. Leonard is an experimental physicist from New Jersey who shares an apartment with colleague and best friend Sheldon Cooper (Jim Parsons). Leonard and Sheldon are named after actor/producer Sheldon Leonard, and Nobel Prize Laureates Robert Hofstadter and Leon Cooper.
Leonard has been described as the straight man of the series. Penny (Kaley Cuoco) is Leonard's next-door neighbor and main love interest, and the teasing of romance between the two of them is a major force driving the series. In 2011, Galecki was nominated for a Primetime Emmy Award for Outstanding Lead Actor in a Comedy Series and a Golden Globe Award for Best Actor – Television Series Musical or Comedy.
Originally from New Jersey, and a graduate of Princeton University, Leonard works as a Caltech experimental physicist mainly working with lasers, who shares an apartment with colleague Sheldon Cooper in Pasadena, California. He is usually seen wearing his characteristic black-framed glasses, low-cut black Converse All Stars sneakers, neutral-colored sweat jackets, brown trousers, and physics-themed t-shirts.
Although Leonard feels at home with his geek colleagues, he wants to be more social. Among the main characters, he generally has the least amount of difficulty interacting with "non-geek" individuals. He is instantly captivated by Penny, and from the beginning sets out to date her.
In early episodes, Leonard is reluctant to let Penny know about his recreational activities (such as Klingon Boggle), as he does not want her to see him as a geek. While certainly a geek by most definitions, he is less socially awkward than Sheldon (who is unable to grasp social norms), Raj (who is unable to talk to women unless he is under the influence of alcohol), and even Howard (who is often seen as "creepy" when flirting with women).
Leonard can also play the cello. As seen in the season 1 episode "The Hamburger Postulate", he and Leslie Winkle practice their instruments and then leave to have sex.
Despite his attempts to not appear to be a geek, Leonard owns, among other things, Star Wars hygiene products, an expansive Superman comic book collection (and wears Clark Kent-type glasses) and a Battlestar Galactica Colonial warrior's flight suit. Early in the series, Leonard tried to get rid of his collectables to impress Penny with his maturity. However, he changed his mind when he saw Penny was already dating another man.
Having lived with Sheldon for several years, Leonard knows him best, and is obligated to explain Sheldon's behavior, and/or serve as a mediator between him and other people in many situations. For instance, when Penny and Sheldon engaged in a fierce dispute, Leonard gave Penny the phone number of Sheldon's mother so she would call her son and scold him for his behavior, thus ending the fight.
Leonard's primary health concern is his lactose intolerance; he avoids eating even small amounts of dairy products, including cheese, yogurt, and butter, for fear of passing gas. According to the (unaired) pilot, he cannot process corn either. Leonard also wears glasses because of myopia, and practically cannot see without them. On one occasion, when his glasses were broken at the movie theater, he had to surreptitiously return to his apartment and retrieve his backup glasses. In order to avoid detection by a sick Sheldon and Penny, who was nursing him, Howard and Raj helped him to navigate through the living room by using a helmet camera. Leonard states in one episode that he suffers from sleep apnea. During his relationship with Leslie Winkle, he tells her that many members of his family have died from heart disease and thus he has a genetic disposition to the condition. At various points in the show, Sheldon also says that Leonard gets carsick unless he sits in the front seat. In season four, it was revealed that Leonard has asthma.