Rick and Morty in The SimpsonsIn the couch gag for the episode "Mathlete's Feat", Rick and Morty were seen crashing through the wall of the house, killing The Simpson family. Morty was terrified to see what he had done and Rick blamed it on him. He told him to go to another dimension with their family portrait and clone them to bring them back. Morty went there and while he did, Rick walked around the house, eating their food and drinking Homer's beer. Ned Flanders walked in and saw what he did and so Rick had to use his Freeze ray to freeze him. When Morty came back, they brought The Simpsons back to life, but they came out looking like mutated versions of themselves that resembled Rick because of their skin color and spiky hair. Knowing that there wasn't really anything else they could do, Rick and Morty quickly fled the scene, leaving them behind. After this, Bart decided that he had had enough and said "No more guest animators!"
In the episode "Springfield Splendor", Dan Harmon made an appearance, teaching a college course on story writing. He was explaining his story circle strategy, shouting "TV should be a circle", before being pelted by pencils and wadded up paper balls, thrown by the students.
The Simpsons in Rick and Morty
One of the background characters in the show has a T-shirt, with a depiction of a blue Bart Simpson head on it.
A second Bart Simpson reference appears in S03E02 "Rickmancing the Stone" on the shirt of a post apocalyptic zombie.
Rick references the Simpsons "Treehouse of Horror" episodes in "Morty's Mind Blowers" when saying that not all the vials have names, unlike the parts of the "Treehouse of Horror" specials.