Falsettos is a sung-through musical with a book by William Finn and James Lapine, and music and lyrics by Finn. The musical consists of March of the Falsettos (1981) and Falsettoland (1990), the last two installments in a trio of one-act musicals that premiered off-Broadway (the first was In Trousers). The story centers on Marvin, who has left his wife to be with a male lover, Whizzer, and struggles to keep his family together. Much of the first act explores the impact his relationship with Whizzer has had on his family. The second act explores family dynamics that evolve as he and his ex-wife plan his son's bar mitzvah, which is complicated as Whizzer comes down with an early case of AIDS. Central to the musical are the themes of Jewish identity, gender roles, and gay life in the late 1970s and early 1980s.

Falsettos premiered on Broadway in 1992 and was nominated for seven Tony Awards, winning those for Best Book and Best Original Score. The musical was revived on Broadway in 2016 starring Christian Borle, Stephanie J. Block, and Andrew Rannells. The 2016 revival was filmed and adapted for the PBS Live from Lincoln Center television series, and aired on October 27, 2017. The revival was nominated for five Tony Awards, including Best Revival of a Musical. Both the original cast and 2016 revival cast performed at the Tony Awards. Other revivals include tours in Australia and the United Kingdom. The musical was praised by critics for its melodic compositions, humor, character development, and positive portrayal of non-traditional family structures.

Composer William Finn began his theater career with a one-act musical In Trousers (1979), which centers on the character Marvin questioning his sexuality. It was produced twice at Playwrights Horizons off-Broadway, opening in February 1978 and again in December 1979. It was also produced off-Broadway at Second Stage Theater in March 1981. After In Trousers received sharply unfavorable reviews, Finn considered abandoning musical theater and attending medical school. He felt that "if the critic for the Times at that time had been more responsible, it would have been a considerable debut. But as it was, he just said it was junk." Finn struggled in his science classes and discarded his medical school plans, turning back to writing about the character of Marvin.

Finn soon wrote the songs for another one-act musical about Marvin and his family, March of the Falsettos, collaborating with director James Lapine on the book. Actress Alison Fraser, who originated the role of Trina, contributed additional vocal orchestrations. This premiered at Playwrights Horizons in April 1981, ran there through September and moved to the Westside Theatre in October 1981. March of the Falsettos received more positive critical reception than In Trousers: Ellen Pall of The New York Times wrote that Finn's "brilliant form combined with the absolute topicality of his social themes first bowled critics over". In 1989, Finn premiered another musical, Romance in Hard Times, which did not feature any of the characters of Falsettos; it was not a success.

Almost a decade after March of the Falsettos, in the wake of the 1980s AIDS epidemic, Finn followed with Falsettoland. The musical concluded Finn's "Marvin Trilogy" of one-act pieces about Marvin and his circle, beginning with In Trousers and March of the Falsettos. Falsettoland opened at Playwrights Horizons on June 28, 1990, then moved to the Lucille Lortel Theatre, on September 16, 1990, where it closed on January 27, 1991. It won the 1991 Lucille Lortel Award for Outstanding Musical and the 1991 Drama Desk Award for Outstanding Lyrics.

Finn and Lapine then combined March of the Falsettos and Falsettoland to form a full-length show, titled Falsettos, slightly altering them to form a "more unified, more thematically consistent" musical. In writing both acts of Falsettos, Finn prioritized making the audience laugh, believing that provoking laughter is more challenging than garnering tears. Each musical was developed during rehearsals, particularly as Finn is a disorganized writer and composer. Finn often composed songs without a clear idea of where they would fit in the musical; he struggled to decide where to place the bar mitzvah in the action. The idea to set it in the hospital came to him in a dream. According to Stephen Bogardus, who played Whizzer in the original cast of both shows as well as in Falsettos, Lapine came up with the idea to incorporate racquetball scenes in Falsettoland, and he and his racquetball partner, Bogardus, added racquetball terminology into the dialogue. The haftorah read by Jason at his bar mitzvah was originally the same one read at Finn's own bar mitzvah, but he "got bored in the middle of writing it" and added words that he enjoyed musically but are grammatically incorrect in Hebrew. Some songs, including "Four Jews in a Room Bitching", originated with Finn humming improvised melodies while strolling the streets of New York City.

In 1979 in New York City, Marvin, his ten-year-old son Jason, his psychiatrist Mendel, and his lover Whizzer are in the midst of an argument ("Four Jews in a Room Bitching"). Marvin explains the situation: he has left his wife Trina for a man, Whizzer, but no one is happy with his attempts to integrate Whizzer into the family ("A Tight-Knit Family"). At Marvin's suggestion, Trina visits Mendel and explains she is having trouble accepting the end of her marriage and her failure to be a perfect wife. Mendel, instantly attracted to her, tries to reassure her that she is not to blame ("Love Is Blind"). Marvin and Whizzer note that they have very little in common but are intensely attracted to each other and worry that their feelings for one another are fading ("The Thrill of First Love"). In a series of therapy sessions with Mendel, Marvin discusses his relationship with Whizzer, his failed relationship with Trina (with Mendel pressing him for intimate details about her), and his inability to connect with his son ("Marvin at the Psychiatrist - A Three-Part Mini-Opera"). Jason is worried that he will turn out to be gay like his father ("My Father's a Homo"). His parents suggest he see Mendel to deal with his mood swings. Jason refuses to listen to his parents but agrees to go to therapy once Whizzer also recommends it ("Everyone Tells Jason to See a Psychiatrist").

Marvin and Whizzer fight over Whizzer's disdain for monogamy and Marvin's attempt to force him into the role of a housewife ("This Had Better Come to a Stop"). Trina is concerned that Whizzer is taking her place in the family and has a mental breakdown ("I'm Breaking Down"). Trina requests that Mendel provide in-home therapy for Jason ("Please Come to Our House"). Mendel gets to know Trina and Jason through these sessions ("Jason's Therapy"). Mendel clumsily proposes to Trina ("A Marriage Proposal"). She accepts, sparking jealousy in Marvin ("A Tight-Knit Family - Reprise"). Trina is frustrated with the male-dominated world she lives in and the immaturity of the men around her ("Trina's Song"). The four sing a hymn to masculinity, the three adults singing in falsetto to match Jason's unbroken voice ("March of the Falsettos"). Trina collects herself and calms down from her frustration ("Trina's Song - Reprise").