There is a DJ known as Nocturnal Ned who works at Arnold's hometown radio station MJZZ. Nocturnal Ned is actually a caricature of "Hey, Arnold!" creator Craig Bartlett, who also provides his voice in various episodes.
In episode "World Records", one of the records in Arnold's book is set by a fictional person named Frannie Caudell. The voice of Helga is Francesca Smith, and the voice of Arnold at the time was Toran Caudell.
Sid has been voiced by two brothers: Sam Gifaldi for most of the series, and one time by Taylor Gifaldi.
The girls of the show are based on girls that creator Craig Bartlett liked as a kid.
The city that Arnold lives in is a combination of Seattle, Brooklyn, and Portland. There are possible hints that the city is called Hillwood. For example, the Hillwood Medical Center or Hillwood Zoo.
The character of Mr. Green the butcher is based on Ernest Borgnine's character in the 1955 film Marty.
Arnold's last name is Shortman.
The name of the boarding house is Sunset Arms Boarding Home.
The sign outside the Sunset Arms Boarding House reads "No Children, Pets OK". This sign can be seen in various places in other episodes, most noticeably at the pool Arnold and Gerald unsuccessfully get into in the heat wave episode.
Ruth P. McDougal was originally Arnold's love interest, and as Arnold talks about her, Gerald constantly tells him that it's hopeless because she's a sixth-grader. However, as Lila was introduced, Ruth was phased out.
This series was originally supposed to end with three half-hour episodes. These episodes became Hey Arnold! The Movie.
Oskar Kokoschka's name is taken directly from a turn of the 20th century Viennese painter, though their personalities are completely opposite.
In the Halloween episode where Arnold and his friends fake an alien invasion, the name of the place where the "U.F.O." (actually the water tower with Christmas lights) landed is Well's Ridge. This is a reference to Orson Welles, who did a fake radio broadcast in which aliens landed on Earth.
The character Eugene Horowitz gets his name from Michael Landon's birth name, Eugene Orowitz.
Arnold's grandfather drives a Packard.
Many stores in the city are named after the writer's and director's in the city - such as Viksten's Pharmacy and many others.
What actually happened to Arnold's parents was never revealed
When Stinky starts recording TV commercials, the studio has a gate that reads "Snee-Oosh Studios." This is the actual name of the studio that worked on the show.
This was the first animated series on Nickelodeon to use children as voice actors.
According to the show's creator, the show takes place in Washington state. Many references to areas and landmarks are made throughout the series.
Arnold's teacher Mr. Simmons is a homosexual according to the show's creators.
The entire episode is based on the classic "The War of the Worlds" radio broadcast from 1938 by Orson Welles and the Mercury Theatre, which caused many listeners to believe that Martians had really invaded Earth. The character of Douglas Cain in this episode is based on Welles and has a name similar to that of the protagonist of Welles' most famous film, Citizen Kane. Ironically, the Orson Welles stand-in in this episode is one of the many convinced of the alien invasion.
When Grandpa is reading the journal, he gets to a part that says, "Stella. What a beautiful name." He asks Arnold what Stella means and the latter says he doesn't know. Stella is a Latin name meaning "star".
"The Journal" was supposed to lead up to a Hey Arnold movie called "The Jungle Movie." But due to many problems, including ones surrounding the Hey Arnold movie released in 2002, The Jungle Movie was never put into production.