Ha Ha! is Nelson's catchphrase and distinctive laugh. Nelson says the laugh with the emphasis on the first "Ha," so it sounds like "HA-ha!"
Nelson usually uses the laugh when something bad happens to another character. Sometimes, after he uses the laugh, he says why he is laughing at the person. For example, he says, "Ha ha! Your dad's not handy!" to Bart after his ten-speed bike (which Homer had assembled) fell apart while Bart was riding it.
Sometimes, Nelson laughs for so long at a time that his voice gets really tired and croaky, which was shown in the Simpsons Movie, when he laughs at Bart..
When Nelson laughs at Martin shortly after punching him in the gut when the latter's birthday party was forced to end early due to the guests falling ill from food poisoning from oysters served by Mrs. Prince, Nelson proceeds to do his trademark laugh, only for his second "ha" to be drawn out into more of a pained groan as he himself succumbs to food poisoning.
In “Bart the General”, Nelson's first appearance, he laughs with three "Ha"s, particularly when Bart imagines Nelson as a giant who eventually eats him up, and in “The War of the Simpsons” he laughs like this to Grampa Simpson.
Sometimes, Nelson will start the laugh, and then partway through realize that the joke is actually on him, and the laugh comes out sounding like a question: "Ha-Ha?" 
The Irish Nelson says "Har! Har!"
When Bart jumps on the see-saw and the rubbish bag on the other side lands on him, Nelson says "See-Saw" in the same sort of way as "Ha Ha!.
Nelson trips up Facebook founder Mark Zuckerberg in a roller-skating rink. Zuckerberg falls and hurts himself and updates his status on Facebook to "in severe pain". Nelson replies, posting "Haw Haw!"
Uses by Other Characters of Simpsons
- Mrs. Muntz has been known to use the laugh.
- Cypress Hill member B-Real says "Ha-ha!" when he overhears Marge referring to Bart wetting his pants at the Christmas play. 
- Lisa also says it in the episode The Haw-Hawed Couple after discovering that Bart and Nelson are best friends.
- Nelson laughs at a tall man driving a Volkswagen Beetle, who turns the tables on him. The man makes Nelson walk down the street with his pants pulled down around his ankles, while everyone in Springfield laughs at him.
- Episode – "Bart the General"
- Episode – "The War of the Simpsons"
- Episode – "The Otto Show"
- Episode – "Duffless"
- Episode – "Marge in Chains"
- Episode – "Treehouse of Horror IV"
- Episode – "Bart Gets Famous"
- Episode – "Lady Bouvier's Lover"
- Episode – "A Star is Burns"
- Episode – "Who Shot Mr. Burns? (Part One)"
- Episode – "El Viaje Misterioso de Nuestro Jomer (The Mysterious Voyage of Homer)" (variation)
- Episode – "22 Short Films About Springfield"
- Episode – "Saturdays of Thunder"
- Episode – "Bart on the Road"
- Episode – "Grade School Confidential"
- Episode – "Marge Simpson in: "Screaming Yellow Honkers""
- Episode – "The Parent Rap"
- Episode – "The Strong Arms of the Ma"
- Episode – "Rome-Old and Julie-Eh" (variation)
- Episode – "Simpson Tide"
- Episode – "Margical History Tour" (variation)
- Episode – "You Kent Always Say What You Want"
- – The Simpsons Movie
- Episode – "In the Name of the Grandfather" (variation)
- Episode – "Boy Meets Curl" (variation)
- Episode – "Postcards From the Wedge"
- Episode – "Lisa Simpson, This Isn't Your Life"
- Episode – "How Munched Is That Birdie in the Window?"
- Episode – "Moe Letter Blues" (opening sequence)
- Episode – "Loan-a Lisa"
- Episode – "Duffless"
- Episode – "King Of The Hill"
- Episode – "Simprovised"
- Episode – "Dad Behavior"
- ↑ I, (Annoyed Grunt)-Bot
- ↑ 2.0 2.1 The Simpsons Movie
- ↑ Grade School Confidential
- ↑ El Viaje Misterioso de Nuestro Jomer (The Mysterious Voyage of Homer)
- ↑ Margical History Tour
- ↑ Boy-Scoutz 'n the Hood
- ↑ Rome-Old and Julie-Eh
- ↑ Boy Meets Curl
- ↑ In the Name of the Grandfather
- ↑ The Debarted
- ↑ "Loan-a Lisa"
- ↑ Homerpalooza
- ↑ 22 Short Films About Springfield