His full name is Aloysius Snuffleupagus, although his friends call him Snuffy. He is eternally 4 years old, and celebrates his birthday on August 19. Snuffy's best friend is Big Bird, who he affectionately calls "Bird." His favorite foods are cabbage and spaghetti. In the fifth season premiere, he revealed that he also drinks sasafrass tea. He has a little sister named Alice and a friend named Rosalyn.
Mr. Snuffleupagus first appeared on Sesame Street in episode 0276, the third season premiere. When he first appeared, many of the adults assumed that he was Big Bird's imaginary friend, due to a series of coincidences and near-miss encounters that continually kept Snuffleupaguses and humans apart. Big Bird would often try to find ways for Snuffy to meet the adults, but something would always cause Snuffy to leave before the humans could see him. Sometimes, all it would take for the adults to see Mr. Snuffleupagus would be to turn their heads, yet they usually wouldn't do so until Snuffy had already gone.
Various kids could see Mr. Snuffleupagus, some Muppets saw him, and even a few celebrities (including Judy Collins, in a fantasy sequence) saw him, but the major human characters never believed his existence until episode 2096 in 1985.
Mr. Snuffleupagus became real to the entire cast for a few reasons. One was because the writers were running out of new ways to have Snuffy just barely miss meeting them. Another factor was increased concerns that the adults' refusal to believe Big Bird's claims of his friend's existence would discourage children from sharing important things with their parents. 
It takes two people to perform Mr. Snuffleupagus. The person who performs the front also performs the voice. Jerry Nelson was the first to play the character. Most sources (including Sesame Street Unpaved and Sesame Street: A Celebration - 40 Years of Life on the Street) state that back problems caused by the physical stress of the performance forced him to bow out, but in a 2009 interview Jerry Nelson gave a different explanation for giving up the role: "I was not loathed to give that character up. But the reasons for giving it up were because at that time we were doing The Muppet Show and he was a real part of the show, and they needed his presence. So they asked if I’d mind giving it up." Michael Earl Davis took over for three seasons (1978–1980), before Marty Robinson became the permanent voice and face of Snuffy. The back of Mr. Snuffleupagus has been performed by Richard Hunt, Brian Muehl, Frank Kane, and Peter Friedman, but since 1979, the regular back-end has been Bryant Young.
In the early years of Snuffy's Sesame Street appearances, his entrances were scored by a low-range brass musical cue (composed by Joe Raposo). A modified version of this theme appeared on various Sesame Street albums. Also, during his first appearances in 1971, Snuffy spoke in a sad-sounding tone of voice. Later, the sad tone was dropped and Snuffy began to speak in a more neutral tone of voice that could sometimes be cheerful. In his debut season, he also had some strange topics on the show, such as being afraid of fruit and wanting to move in with Big Bird. Also, sometimes he would answer questions with statements that did not make sense. Such as in the closing of his premiere episode Big Bird asks "Where did you go?" and he replies "Well I suppose so."
Mr. Snuffleupagus' Family
Mr. Snuffleupagus lives with his family in a cave located at 456 Snuffle Circle.
- Mommy Snuffleupagus (also known as "Mommy Snuffle")
- Daddy Snuffle
- Alice Snuffleupagus (younger sister)
- Aunt Agnes Snuffleupagus
- Granny Snuffle
- Abigail Snuffleupagus (cousin)
- Señor Snuffleupago (cousin)
- Uncle Abe
In 1992, the producers of Sesame Street tested an episode in which Snuffy's parents get a divorce. After a series of arguments, Daddy Snuffle moved out of the cave, and Snuffy and Alice had to deal with the emotional fallout of a breakup. The episode didn't test well with children, and the producers decided to shelve it. In the context of the show, Mommy and Daddy Snuffleupagus are still married. For more information, see "Snuffy's Parents Get a Divorce".
- Episode 0276: Snuffy first appears
- Episode 0331: Snuffy celebrates his 2nd birthday
- Episode 1090 - Episode 1095: Trip to Hawaii
- Episode 1800: Cousin Abigail visits
- Episode 2040: Snuffy Goes to the Movies
- Episode 2096: The adults meet Snuffy for the first time
- Episode 2208: Big Bird and Snuffy remember how they met
- Episode 2477: Snuffy plays with Alice
- Episode 3821: Snuffy draws a huge picture for Gina
- Episode 4069 - Episode 4070: Snuffy's Invisible
- Episode 4088: Snuffy wants to be a cloud
- Episode 4091: Snuffy feels he's too big
- The original Mr. Snuffleupagus puppet had bright yellow eyes, immobile eyelids, and yellow eyelashes. He was redesigned in Season 4, gaining white eyes, eyelids, and eyelashes, to look more like he does today.
- For a time, his catchphrase was "Ohhh, dear." Some of the puppeteers still use the phrase when they are playing around with their puppets backstage.
- When not filming, the producers of Sesame Street hang Snuffy from the studio ceiling, wrapped in a pink blanket. They call him the Big Pink Moon. They place him there because the suit is so enormous that there is no room for him in storage.
- Snuffy is a big fan of opera music, and at one point attended daily rehearsals at Metropolitan Opera House in New York City. He even performed a light-opera rendition of the alphabet with Judy Collins.
- Similar to how Big Bird often mispronounced Mr. Hooper's name as Mr. Looper, Snuffy often mispronounced Mr. Handford's name as Mr. Handfoot.
- Sometime before seeing for themselves that he was real, Maria, Gordon, and Linda all decided to believe that he was real despite the lack of evidence, while Uncle Wally initially believed that he was real, claiming to have encountered various Snuffleupaguses over the course of his travels.
- Martin Robinson once said about Snuffy, "He's not invisible, He just has bad timing!" 
- In one episode, Snuffy tap danced with Maurice and Gregory Hines, known as the Hines Brothers, who were at that time appearing in the Broadway musical Eubie!
- His shoe size (or at least his roller skate size) is 65 triple G.
- The Snuffy puppet and his facial movements are operated entirely from within the puppet. There is a long stick that moves his eyes around, a long string to pull down his eyelids, and a handle to pull down the lower part of his mouth, as the size of his mouth makes it practically impossible for a puppeteer to operate it like a normal puppet. The interior was first publicly shown in the Sesame Family Robinson blog post "HandyMan in Snuffleupagus". In another post, Martin P. Robinson explained that in between takes, one of the cameramen fan air through the mouth of the puppet to cool off the puppeteers. Robinson also explained it was a lot easier to perform the puppet when it was warm as he was less likely to pull muscles, although on some days he would go through 3 or 4 T-shirts.
- The puppet is so big that when producers had to move it to different locations during filming of the Hawaii episodes, it was hung down from a helicopter.
- In an episode featuring Snuffy's cousin Snuffleupago, the regular Snuffy puppet was given a sombrero. However, during filming, the weight of the sombrero caused the aluminum frame backpack the puppeteers use to hold Snuffy gave way, causing a huge wall of fur to hold down Marty Robinson and Bryant Young. Luckily, no one was hurt, and the puppet was rebuilt in 20 minutes. Eventually, the backpack was replaced.
- The Snuffy puppet was used for Daddy Snuffle, unaltered, in Snuffy's Parents Get a Divorce, as neither characters appeared in the same scene together.
- When Snuffy was first born, he was 572 pounds and 5 ounces.
Before his existence was revealed to the grown-ups on Sesame Street, there were many instances when the adults almost saw Mr. Snuffleupagus, only to just barely miss him. Some even heard him talk, but thought they were imagining things. Here are some of the many close encounters between Mr. Snuffleupagus and others.
- In Episode 0323, Big Bird takes a picture of Mr. Snuffleupagus, but when he shows the picture to the adults, they think that they're looking at an old rug and a furry rope.
- In Episode 0536, Big Bird and the adults play a game of Follow the Leader. Big Bird wants them to wait for Mr. Snuffleupagus to arrive, but they refuse to wait, thinking that otherwise they'll never play. After they start playing, Mr. Snuffleupagus shows up and takes the last place in line. David is the leader, and tells everyone to close their eyes, which causes them to miss Snuffy. Snuffy gets tired and leaves before the game is over.
- In Episode 0746, Mr. Snuffleupagus attempts to introduce himself to Mr. Hooper. Unfortunately, Mr. Hooper is wearing headphones and reading the newspaper, so he can't see or hear Snuffy. Big Bird comes in and finds Snuffy taking a nap next to Mr. Hooper, but Mr. Hooper still insists that there's no such thing as a Snuffleupagus.
- In Episode 0796, Mr. Snuffleupagus walks down Sesame Street, counting the number of people who don't see him -- including Susan, Gordon, Mr. MacIntosh, Cookie Monster (who gets his head stuck inside a bag of cookies), Bob, David, Maria, Mr. Hooper (who loses his glasses when Snuffy walks in front of him), Willy, and Luis (who holds a stack of boxes that block his vision).
- In Episode 0807, Mr. Snuffleupagus comes to Sesame Street wearing black-and-white overalls. Bob has dozed off on the stoop, and wakes up just as Snuffy is leaving. Bob gets a glimpse of Snuffy as he's waking up. However, when Bob tells Big Bird about seeing the Snuffleupagus, he mentions the black and white stripes, and Big Bird thinks Bob must have been imagining things.
- In Episode 0858, Mr. Hooper has lost his glasses again. Groping around for them, he feels Snuffy's fur, but he thinks that he's touching the furry scarf that Maria lost.
- In Episode 1186, David thinks he hears Big Bird talking to Mr. Snuffleupagus about going to the movies. He convinces the other adults to come to Big Bird's nest. When they get there, Snuffy is gone, and they decide that David must have been hearing things.
- In Episode 1800, Snuffy takes his broken toaster to the Fix-It Shop. He speaks with Maria, who's too busy to look up; she tells her customer to leave the toaster for her. When she looks up, she's amazed to find a Snuffleupagus-sized toaster. Maria shows Olivia the huge toaster, and wonders if she spoke to Mr. Snuffleupagus. Olivia thinks that Big Bird must be playing a trick. They wait to see if Snuffy comes back to pick up the toaster -- but Big Bird comes for it instead; Snuffy asked him to pick up the toaster for him.
- In Episode 1836, Forgetful Jones is manning a water station for the New York City marathon. Snuffy gets some water from Jones. When Bob comes by a moment later, Forgetful Jones forgets all about his meeting with Snuffy.
- In Episode 2040, Big Bird, Snuffy, and some of the adults go to the movies. Big Bird and Snuffy are sitting in the row behind those of the adults who attend the movie. Big Bird asks Bob several times to just quickly turn around and see Snuffy, but he refuses to, saying he doesn't want to miss any of the movie.
- In Episode 1957, Big Bird decides that it's a good day for Snuffy to meet his friends. Big Bird asks Snuffy to stay at his nest area while he looks for friends, and finds David and Olivia, who have to go vote for a candidate (it's election day). Big Bird wants to learn more about voting, so they take him to the voting area and show him how things work. Big Bird then remembers that Snuffy is still waiting to meet them, and runs back to his nest area to take Snuffy there, to meet them and also learn about voting. When they get back, Olivia has already left, and David is in a voting booth, refusing to come out while he's voting. He also explains that Big Bird can't vote because in order to vote, one has to be registered and 18. Snuffy realises that his mommy is over 18, and leaves to see if she is aware that it's election day, stating that it's more important than meeting Big Bird's friends. David then comes out, having heard Snuffy's voice, but still doesn't believe that Snuffy was actually there. He then tells Big Bird that there's another important qualification for voting, besides being 18 and registered: "You also have to be real!"
- In one episode, Snuffy brings Fluffy back home to Oscar's trash can after a visit. Telly Monster, who had been searching for Fluffy, sees Snuffy and becomes convinced that he saw him. Telly runs to Hooper's Store to tell the adults, who think he was imagining Snuffy. Telly brings the others to Oscar's can to see if they could see Snuffy, but Snuffy left before they got there, convincing Telly that he was imagining him. But then Snuffy sticks his nose out of the trash can, making Telly wonder if he was imagining that.
- In Don't Eat the Pictures, Snuffy walks by the museum guard, played by Paul Dooley, and mumbling to himself, but just beforehand, the guard had removed his glasses and was now wiping them and not paying attention.
- In one scene (which was included in A Walking Tour of Sesame Street), Big Bird takes the adults to a location to meet Snuffy. While in front of a fountain, Big Bird spots Snuffy behind a fountain in the distance and tells the adults to turn around, but the adults initially refuse and laugh at the situation. When they finally decide to turn around, the fountain turns on and the water obstructs their view of Snuffy. They all then walk away before the fountain water turns off.
Books that focus on Snuffy include:
- Don't Be Shy (1987)
- Meet Mr. Snuffle-upagus (1988)
- The Day Snuffy Had the Sniffles (1988)
- Bert and the Magic Lamp (1989): "Life-Styles of the Large and Furry"
- See You Later, Mashed Potater! (1990)
- The Sesame Street 1, 2, 3 Storybook (1973)
- Grover and the Everything in the Whole Wide World Museum (1974)
- The Sesame Street ABC Storybook (1974)
- The Great Sesame Street ABC Hunt (1975)
- The Sesame Street A to Z Coloring Book (1976)
- The Tool Box Book (1980)
- I Have a Friend (1981)
- Prairie Dawn's Upside-Down Poem (1981)
- Put & Play Magic Show (1981)
- Big and Little Stories (1982)
- Don't Cry, Big Bird (1983)
- There's No Place Like Home (1983)
- I Think That It Is Wonderful (1984)
- Big Bird Joins the Carnival (1985)
- Big Bird's Book of Rhymes (1985)
- Ernie's Finish the Picture (1985)
- Follow That Bird Activity Book (1985)
- Follow That Bird coloring book (1985)
- Grover's Book of Colors (1985)
- ABC (1986)
- A My Name Is Annabel (1986)
- Sesame Street ABC (1986)
- Colors (1987)
- A Rainy Day on Sesame Street (1987)
- Shape Up! (1987)
- A New Playground on Sesame Street (1988)
- A My Name Is Alice (1989)
- Museum of Monster Art (1990)
- Sleep Tight! (1991)
- We're Counting on You, Grover! (1991)
- Grover's 10 Terrific Ways to Help Our Wonderful World (1992)
- Sesame Street Stays Up Late (1995)
- It's Not Easy Being Big! (1998)
- The Fix-It Shop (2001)
- Clap Your Hands! (2002)
- Red or Blue, I Like You! (2005)
- Let's Compare Sizes (2006)
- Abby Cadabby's Rhyme Time (2007)
- All About Me! (2007)
- Friendly, Frosty Monsters (2007)
- Music Player Storybook (2007)
- What Makes You Giggle? (2007)
- Our Kind of Bird (2008)
- Storybook ABCs (2008)
- Love, Elmo (2009)
- Murray's First Book of Words (2010)
- Knickerbocker doll
- Snuffle-Upagus Dingy (Hasbro die-cast car)
- Applause PVCs -- many Mr. Snuffleupagus PVCs released
- Mr. Snuffleupagus bean
- Tyco PVC figure