At ten years old, Bart is the eldest child and only son of Homer and Marge, and the brother of Lisa and Maggie. Bart's most prominent character traits are his mischievousness, rebelliousness and disrespect for authority. He has appeared in other media relating to The Simpsons; including video games, The Simpsons Movie, The Simpsons Ride, commercials, and comic books; and inspired an entire line of merchandise.

In casting, Nancy Cartwright originally planned to audition for the role of Lisa, while Yeardley Smith tried out for Bart. Smith's voice was too high for a boy, so she was given the role of Lisa. Cartwright found that Lisa was not interesting at the time, so instead auditioned for Bart, which she thought was a better role. Hallmarks of the character include his chalkboard gags in the opening sequence; his prank calls to Moe the bartender; and his catchphrases "Eat my shorts", "¡Ay, caramba!", and "Don't have a cow, man!"

During the first two seasons of The Simpsons (1989–1991), Bart was the show's breakout character and "Bartmania" ensued. Bart Simpson T-shirts sporting various slogans and catchphrases became popular, selling at a rate of a million per day at their peak. The song "Do the Bartman" became a number one charting single and the seventh best-selling song of 1991 in the United Kingdom. Bart's rebellious attitude and pride at underachieving caused many parents and educators to cast him as a bad role model for children. A T-shirt reading "I'm Bart Simpson. Who the hell are you?" was banned in several public schools. Around the third season, the series started to focus more on the family as a group, although Bart remains one of the most prominent characters on the series. Time named Bart one of the 100 most important people of the 20th century, and he was named "entertainer of the year" in 1990 by Entertainment Weekly. Nancy Cartwright has won several awards for voicing Bart, including a Primetime Emmy Award in 1992 and an Annie Award in 1995. In 2000, Bart, along with the rest of his family, was awarded a star on the Hollywood Walk of Fame.

The Simpsons uses a floating timeline in which the characters do not age, and as such the show is always assumed to be set in the current year. In several episodes, events have been linked to specific times, though sometimes this timeline has been contradicted in subsequent episodes. Bart's year of birth was stated in "I Married Marge" (season three, 1991) as being in the early 1980s. He lived with his parents in the Lower East of Springfield until the Simpsons bought their first house. When Lisa was born, Bart was at first jealous of the attention she received, but he soon warmed to her when he discovered that "Bart" was her first word. Bart's first day of school was in the early 1990s. His initial enthusiasm was crushed by an uncaring teacher and Marge became worried that something was truly wrong with him. One day during recess, Bart met Milhouse and started entertaining him and other students with various gestures and rude words. Principal Skinner told him "you've just started school, and the path you choose now may be the one you follow for the rest of your life! Now, what do you say?" In his moment of truth, Bart responded, "eat my shorts". The episode "That 90's Show" (season nineteen, 2008) contradicted much of the backstory's time frame; for example, it was revealed that Homer and Marge were childless in the early 1990s.

Bart's various hobbies include skateboarding, watching television (especially The Krusty the Clown Show which includes The Itchy & Scratchy Show), reading comic books (especially Radioactive Man), playing video games and generally causing mischief. For the duration of the series, Bart has attended Springfield Elementary School and has been in Edna Krabappel's fourth grade class. While he is too young to hold a full-time job, he has had occasional part time jobs. He works as a bartender at Fat Tony's social club in "Bart the Murderer" (season three, 1991); as Krusty the Clown's assistant in "Bart Gets Famous" (season five, 1994); as a doorman in Springfield's burlesque house, the Maison Derrière in "Bart After Dark" (season eight, 1996); and briefly owns his own factory in "Homer's Enemy". (season eight, 1997)

Matt Groening first conceived of Bart and the rest of the Simpson family in 1986, while waiting in the lobby of producer James L. Brooks' office. Groening had been called in to pitch a series of animated shorts for The Tracey Ullman Show, and had intended to present an adaptation of his Life in Hell comic strip. When he realized that animating Life in Hell would require him to rescind publication rights, Groening decided to go in another direction. He hurriedly sketched out his version of a dysfunctional family, naming the characters after members of his own family. For the rebellious son, he substituted "Bart", an anagram of the word brat, for his own name, as he decided it would have been too obvious for him to have named the character Matt.

Bart had originally been envisioned as "a much milder, troubled youth given to existential angst who talks to himself", but the character was changed based on Cartwright's voice acting. Groening has credited several different figures with providing inspiration for Bart: Matt Groening's older brother Mark provided much of the motivation for Bart's attitude. Bart was conceived as an extreme version of the typical misbehaving child character, merging all of the extreme traits of characters such as Tom Sawyer and Huckleberry Finn into one person. Groening describes Bart as "what would happen if Eddie Haskell [from Leave it to Beaver] got his own show". Groening has also said that he found the premise of Dennis the Menace disappointing and was inspired to create a character who was actually a menace. Bart's middle initial J is a "tribute" to animated characters such as Bullwinkle J. Moose and Rocket J. Squirrel from The Rocky and Bullwinkle Show, who received their middle initial from Jay Ward. According to the book Bart Simpson's Guide to Life, Bart's full middle name is JoJo.

Bart made his debut with the rest of the Simpson family on April 19, 1987 in The Tracey Ullman Show short "Good Night". In 1989, the shorts were adapted into The Simpsons, a half-hour series airing on the Fox Broadcasting Company. Bart and the Simpson family remained the main characters on this new show.

The entire Simpson family was designed so that they would be recognizable in silhouette. The family was crudely drawn, because Groening had submitted basic sketches to the animators, assuming they would clean them up; instead, they just traced over his drawings. Bart's original design, which appeared in the first shorts, had spikier hair, and the spikes were of different lengths. The number was later limited to nine spikes, all of the same size. At the time Groening was primarily drawing in black and "not thinking that [Bart] would eventually be drawn in color" gave him spikes which appear to be an extension of his head. The features of Bart's character design are generally not used in other characters; for example, no other characters in current episodes have Bart's spiky hairline, although several background characters in the first few seasons shared the trait.

The basic rectangular shape of Bart's head is described by director Mark Kirkland as a coffee can. Homer's head is also rectangular (with a dome on top), while spheres are used for Marge, Lisa, and Maggie. Different animators have different methods of drawing Bart. Former director Jeffrey Lynch starts off with a box, then adds the eyes, then the mouth, then the hair spikes, ear, and then the rest of the body. Matt Groening normally starts with the eyes, then the nose, and the rest of the outline of Bart's head. Many of the animators have trouble drawing Bart's spikes evenly; one trick they use is to draw one on the right, one on the left, one in the middle, then continue to add one in the middle of the blank space until there are nine. Originally, whenever Bart was to be drawn from an angle looking down so the top of his head was seen, Groening wanted there to be spikes along the outline of his head, and in the middle as well. Instead, Wes Archer and David Silverman drew him so that there was an outline of the spikes, then just a smooth patch in the middle because "it worked graphically."