Elmo's World is a segment that is shown in the American children's television program Sesame Street. It always comes last and premiered on November 16, 1998, as part of a broader structural change to the show. It originally lasted fifteen minutes at the end of each episode. The segment ran until 2009, and then returned in 2017. The segment was designed to appeal to younger viewers and to increase ratings, which had fallen in the past decade. The segment is presented from the perspective of a three-year-old child as represented by its host, the Muppet Elmo, performed by Kevin Clash in the original series and Ryan Dillon in the 2017 reboot.

The segment was developed out of a series of workshops that studied changes in the viewing habits of Sesame Street's audience, and the reasons for the show's lower ratings. Elmo's World used traditional production elements, but had a more sustained narrative. In 2002, Sesame Street's producers changed the rest of the show to reflect its younger demographic and the increase in their viewers' sophistication.

Long-time writer Judy Freudberg came up with the concept of Elmo's World, and writer Tony Geiss and executive producer Arlene Sherman helped develop it. In contrast with the realism of the rest of the show, the segment presented Elmo moving between and combining a live action world and a computer-generated animated world, which looked like "a child's squiggly crayon drawing come to life", with "a stream-of-consciousness feel to it". Elmo's pet goldfish Dorothy and the members of the Noodle family were silent to allow Elmo to do all the talking, and to give children the opportunity to respond to what they saw on the screen. A brief clip from Elmo's World appears in Sesame Street's 2002–2006 intro.

In 2009, Elmo's World temporarily ceased production. It was replaced by Elmo: The Musical in 2012, and returned in 2017. The show is alternated by two more Elmo-centered segments, Elmo & Tango’s Mysterious Mysteries and Nature Explorers, which are both produced entirely with animation.

By the early 1990s, Sesame Street had been on the air for over 20 years and was, as author Michael Davis put it, "the undisputed heavyweight champion of preschool television". The show's dominance began to be challenged throughout the decade by other television shows for preschoolers such as Barney & Friends, Blue's Clues, and Teletubbies, by the growth of the children's home video industry, and by the increase of thirty-minute children's shows on cable. Sesame Street's ratings declined, so the Children's Television Workshop (CTW) (now Sesame Workshop), the organization responsible for putting the show on the air, responded by researching the reasons for their lower ratings.

For the first time since the show debuted, the producers and a team of researchers analyzed Sesame Street's content and structure, and studied how children's viewing habits had changed. The analysis was conducted during a series of two-week-long workshops and was completed in time for the show's 30th anniversary in 1999. The CTW found that although the show was produced for children between the ages of three and five, their viewers had become more sophisticated since its debut and began to watch the show sooner, as early as ten months of age. The producers found that the show's original format, which consisted of a series of short clips similar to the structure of a magazine, was not necessarily the most effective way to hold young viewers' attention. They also found that their viewers, especially the younger ones, lost attention with Sesame Street after 40 to 45 minutes.

The first way the CTW addressed the issues brought up by their research was by lowering the target age for Sesame Street, from four years to three years. In late 1998, they created a new 15-minute segment entitled Elmo's World, hosted by the Muppet Elmo, that was shown at the end of each episode. The segment used traditional elements (animation, Muppets, music, and live-action film), but had a more sustained narrative. Elmo's World followed the same structure each episode, and depended heavily on repetition. It focused on child-centered topics such as balls and dancing, from the perspective of a three-year-old child, and was "designed to foster exploration, imagination, and curiosity". Instead of an adult providing narration, Elmo led the child through the action.

In 2002, Sesame Street's producers went further in changing the show to reflect its younger demographic and increase in their viewers' sophistication. They decided, after the show's 33rd season, to expand upon the Elmo's World concept by, as San Francisco Chronicle TV critic Tim Goodman called it, "deconstructing" the show. They changed the structure of the entire show to a more narrative format, making the show easier for young children to navigate. Arlene Sherman, a co-executive producer for 25 years and one of the creators of Elmo's World, called the show's new look "startlingly different".

Long-time Sesame Street writer Judy Freudberg came up with the idea of creating a segment with "an entirely different format" from the rest of the show during the CTW's workshops, and writer Tony Geiss further developed the idea with her. Freudberg stated that the concept "was radical because we had never veered from that magazine mosaic and had never given any character more than another character to do".

Animator Mo Willems came up with the idea of creating a less-realistic setting compared to the rest of the show. The segment presented Elmo, first portrayed by Kevin Clash and then by Ryan Dillon after its revamp in 2017, moving between and combining two worlds of live action and computer-generated animation, which looked like "a child's squiggly crayon drawing come to life" created by the host, and with "a stream-of-consciousness feel to it". The segment was filmed at a different time than the rest of the season, much of it in front of a blue screen, with animation and digital effects added later. For more complicated shots that showed Elmo's entire body, a puppet called "Active Elmo" was operated with assistance from other puppeteers; the puppet was also filmed in front of a blue screen and edited later. Visual effects for the series were provided by Curious Pictures (1998 to 1999), Protozoa (2000), and Celefex (2001 to 2009).

In addition to Freudberg and Geiss, other writers of Elmo's World included Emily Kingsley and Molly Boylan. The theme song was based upon a song Geiss wrote called "Elmo's Song", with lyrics changed to fit the segment. Writer Louise Gikow and The New York Times called it "a show within a show". Clash called it "a playdate between the child and Elmo", and felt that its intimacy provided an effective teaching tool. He also called it "an instant success". Davis compared Elmo's World with the Saturday Morning children's TV show Pee-wee's Playhouse.

The CTW, as it has done throughout its existence and for all the shows it produced, conducted extensive studies on Elmo's World. They found that the segment had high appeal for children, regardless of their age, sex, and socioeconomic background. Attention and participation such as hand-clapping, moving along with the music, and counting along with the characters increased with repeated viewing.