West Side Story is a musical conceived by Jerome Robbins with music by Leonard Bernstein, lyrics by Stephen Sondheim, and a book by Arthur Laurents.

Inspired by William Shakespeare's play Romeo and Juliet, the story is set in the mid-1950s on the Upper West Side of Manhattan in New York City, then a multiracial, blue-collar neighborhood. The musical explores the rivalry between the Jets and the Sharks, two teenage street gangs of different ethnic backgrounds. The Sharks, who are recent migrants from Puerto Rico, and the Jets, who are white, vie for dominance of the neighborhood, and the police try to keep order. The young protagonist, Tony, a former member of the Jets and best friend of the gang's leader, Riff, falls in love with Maria, the sister of Bernardo, the leader of the Sharks. The dark theme, sophisticated music, extended dance scenes, tragic love story, and focus on social problems marked a turning point in musical theatre.

The original 1957 Broadway production, directed and choreographed by Robbins, marked Sondheim's Broadway debut. It ran for 732 performances before going on tour. The production was nominated for six Tony Awards, including Best Musical, in 1958, winning two. The show had an even longer-running West End production, a number of revivals, and international productions. A 1961 musical film adaptation, co-directed by Robert Wise and Robbins, was nominated for eleven Academy Awards and won ten, including Best Picture. A 2021 film adaptation, directed by Steven Spielberg was also nominated for the Academy Award for Best Picture, along with six additional nominations, winning for Best Supporting Actress.

In 1949, Jerome Robbins approached Leonard Bernstein and Arthur Laurents about collaborating on a contemporary musical adaptation of Romeo and Juliet. He proposed that the plot focus on the conflict between a Catholic family and a Jewish family living on the Lower East Side of Manhattan, during the Easter–Passover season; the girl has survived the Holocaust and emigrated from Israel. The conflict was to be centered on antisemitism of the Catholic "Jets" towards the Jewish "Emeralds". Eager to write his first musical, Laurents immediately agreed. Bernstein wanted to present the material in operatic form, but Robbins and Laurents resisted the suggestion. They described the project as "lyric theater", and Laurents wrote a first draft he called East Side Story. Only after he completed it did the group realize it was little more than a musicalization of themes that had already been covered in plays like Abie's Irish Rose. When Robbins opted to drop out, the three men went their separate ways, and the piece was shelved for almost five years.

In 1955, theatrical producer Martin Gabel was working on a stage adaptation of the James M. Cain novel Serenade, about an opera singer who comes to the realization he is homosexual, and he invited Laurents to write the book. Laurents accepted and suggested Bernstein and Robbins join the creative team. Robbins felt that if the three were going to join forces, they should return to East Side Story, and Bernstein agreed. Laurents, however, was committed to Gabel, who introduced him to the young composer/lyricist Stephen Sondheim. Sondheim auditioned by playing the score for Saturday Night, his musical that was scheduled to open in the fall. Laurents liked the lyrics but was not impressed with the music. Sondheim did not care for Laurents' opinion. Serenade ultimately was shelved.

Laurents was soon hired to write the screenplay for a remake of the 1934 Greta Garbo film The Painted Veil for Ava Gardner. While in Hollywood, he contacted Bernstein, who was in town conducting at the Hollywood Bowl. The two met at The Beverly Hills Hotel, and the conversation turned to juvenile delinquent gangs, a fairly recent social phenomenon that had received major coverage on the front pages of the morning newspapers due to a Chicano turf war. Bernstein suggested they rework East Side Story and set it in Los Angeles, but Laurents felt he was more familiar with Puerto Ricans in the United States and Harlem than he was with Mexican Americans and Olvera Street. The two contacted Robbins, who was enthusiastic about a musical with a Latin beat. He arrived in Hollywood to choreograph the dance sequences for the 1956 film The King and I, and he and Laurents began developing the musical while working on their respective projects, keeping in touch with Bernstein, who had returned to New York. When the producer of The Painted Veil replaced Gardner with Eleanor Parker and asked Laurents to revise his script with her in mind, he backed out of the film, freeing him to devote all his time to the stage musical. Bernstein and Laurents, who had been blacklisted for alleged communist activities, worked with Robbins even though he had cooperated with the House Un-American Activities Committee.

In New York City, Laurents went to the opening night party for a new play by Ugo Betti. There he met Sondheim, who had heard that East Side Story, now retitled West Side Story, was back on track. Bernstein had decided he needed to concentrate solely on the music, and he and Robbins had invited Betty Comden and Adolph Green to write the lyrics, but the team opted to work on Peter Pan instead. Laurents asked Sondheim if he would be interested in tackling the task. Initially he resisted, because he was determined to write the full score for his next project (Saturday Night had been scrapped). But Oscar Hammerstein convinced him that he would benefit from the experience, and he accepted. Meanwhile, Laurents had written a new draft of the book changing the characters' backgrounds: the male lead, once an Irish American, was now of Polish and Irish descent, and the formerly Jewish female lead had become Puerto Rican.

The original book Laurents wrote closely adhered to Romeo and Juliet, but the characters based on Shakespeare's Rosaline and the parents of the doomed lovers were eliminated early on. Later the scenes related to Juliet's faking her death and committing suicide also were deleted. Language posed a problem; profanity was uncommon in the theater at the time, and slang expressions were avoided for fear they would be dated by the time the production opened. Laurents ultimately invented what sounded like real street talk but actually was not: "cut the frabba-jabba", for example. Sondheim converted long passages of dialogue, and sometimes just a simple phrase like "A boy like that would kill your brother", into lyrics. With the help of Oscar Hammerstein, Laurents convinced Bernstein and Sondheim to move "One Hand, One Heart", which he considered too pristine for the balcony scene, to the scene set in the bridal shop, and as a result "Tonight" was written to replace it. Laurents felt that the building tension needed to be alleviated in order to increase the impact of the play's tragic outcome, so comic relief in the form of Officer Krupke was added to the second act. He was outvoted on other issues: he felt the lyrics to "America" and "I Feel Pretty" were too witty for the characters singing them, but they stayed in the score and proved to be audience favorites. Another song, "Kid Stuff", was added and quickly removed during the Washington, D.C., tryout when Laurents convinced the others it was helping tip the balance of the show into typical musical comedy.