The fictional James Buchanan High School is based on New Utrecht High School in Brooklyn, which Gabe Kaplan actually attended and which appears in the opening credits.

Many of the show's characters were based on real people Kaplan knew during his years as a remedial student, several of whom he had previously described in his stand-up routine 'Holes and Mellow Rolls'; for example, Vinnie Barbarino was inspired by Eddie Lecarri and Ray Barbarino, and Juan Epstein was partly inspired by a real person nicknamed 'Epstein the Animal'.

The show was originally going to be titled simply 'Kotter', but the name was changed because the producers wanted it to match the lyrics of the theme song 'Welcome Back'.

John Travolta's role as Vinnie Barbarino became his breakthrough part, though by the fourth season he had already starred in Carrie, Saturday Night Fever, and Grease, and appeared in only 10 episodes as a 'special guest star' earning $2,000 per episode.

Debralee Scott's character Rosalie 'Hotsie Totsie' was phased out at the end of season one because Scott was cast to co-star in Norman Lear's syndicated comedy Mary Hartman, Mary Hartman.

For the final season, producers deliberately sought a Southern heartthrob character (Beau De LaBarre) who would not be a direct copycat of the 'Italian Stallion' Travolta type, aiming to boost the show's appeal in the South and retain female viewers without simply cloning Barbarino.

Although Welcome Back, Kotter aired on ABC, some earlier episodes were actually shot at NBC's Burbank studios.

In Boston, the ABC affiliate WCVB-TV initially refused to air the first season because the city was in the midst of a tense school busing controversy, and the station feared the show's integrated-classroom premise would inflame tensions; it relented and began airing the show from its fifth episode after it proved a ratings hit.

Because of concerns from teachers in other cities about how the profession would be portrayed, producers allowed a union representative on set to monitor the show, a decision Gabe Kaplan opposed, joking to a reporter that no junkman was on the set of Sanford and Son to protect junkmen's reputations.

Groucho Marx was scheduled for a brief walk-on cameo in one episode and arrived on set, but was deemed too sick to appear; photographs of him with the cast were taken but not released at the time, only surfacing on the internet decades later.

Robert Hegyes, a fan of the Marx Brothers like Kaplan, claimed he suggested that the Sweathogs be modeled after the Marx Brothers to help defuse censors' concerns about depicting juvenile delinquency.

The show's popularity spawned extensive merchandising, including lunchboxes, dolls, trading cards, comic books, novels, and a board game called 'Up Your Nose With a Rubber Hose', advertised in a commercial featuring Steve Guttenberg and Thomas Carter as young Sweathog look-alikes.

The theme song 'Welcome Back' was written and performed by John Sebastian, former frontman of the Lovin' Spoonful, and became a No. 1 hit in spring 1976.

Two spin-off attempts largely failed: Mr. T and Tina, starring Pat Morita as an inventor character introduced in a Kotter episode, lasted only five episodes, while a backdoor pilot for a Horshack-centered spin-off never went beyond the pilot stage.