The Marvin Trilogy is a trilogy of one-act musicals created by William Finn. It consists of In Trousers (1978), March of the Falsettos (1981), and Falsettoland (1990). The musicals center on Marvin, who has left his wife to be with a male lover, Whizzer, and struggles to keep his family together. The first act of the trilogy (In Trousers) focuses on Marvin's life as an adolescent, the influential women of his life, and his struggle to accept his sexual identity. Much of the second act (March of the Falsettos) explores the impact his relationship with Whizzer has had on his family. The third act (Falsettoland) focuses on how family dynamics evolve as Marvin and Trina plan for their son's Bar Mitzvah. Central to the musical are the themes of Jewish identity, gender roles, and gay life in the late 1970s and early 1980s.

March of the Falsettos and Falsettoland were later combined into one musical titled Falsettos with music and lyrics by Finn and a book by Lapine. When referring to Falsettos, March of the Falsettos is thought of as the first act, as opposed to In Trousers, which is act one in the Marvin Trilogy. The 2016 Broadway revival of Falsettos is the most widely well-known rendition of any play in the Marvin Trilogy. As for the first musical in the Marvin Trilogy, In Trousers is the most obscure musical of the three.

Composer William Finn began his theater career with the one-act musical In Trousers (1978). 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. 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, March of the Falsettos (1981), collaborating with director James Lapine on the book. This show premiered at Playwrights Horizons in April 1981 and ran through September before moving to the Westside Theatre in October 1981. Almost a decade after March of the Falsettos, in the wake of the ravages 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. 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.

The one-act musical centers on Marvin, who has a wife and child. He recalls the past relationships he shared with, among others, his high school sweetheart and Miss Goldberg, his English teacher who let him play Christopher Columbus in the school play, and then reveals he prefers to be with men. Torn between his natural inclination and his desire not to upset his family life as he knows it, Marvin ultimately makes the decision he feels is best for him.

In 1979 New York City, Marvin tells the audience that he has left his family, his wife Trina and son Jason, for a male lover named Whizzer. Marvin recommends that Trina get help with his psychiatrist, Mendel. The two end up getting engaged, much to Marvin's dismay. Meanwhile, he and Whizzer have several relationship problems and have an on-again, off-again bond. When Whizzer wins a game of chess, Marvin promptly breaks up with him for the final time. At the same time, Jason battles his questions about his parents' divorce and individual relationships. As a result, Trina sends him to therapy with Mendel which seems to help the child. In the final scene of the show, Marvin gets an invitation to Trina and Mendel's wedding, which makes him mad enough to hit Trina. He apologizes to Jason for his behavior and promises to stay calm.

Two years later, Trina and Mendel are married, Marvin and Whizzer are still separated, and Jason is about to turn thirteen. Also in the group of friends are two lesbians who live next door to Marvin, medical doctor Charlotte and aspiring chef Cordelia. Trina and Marvin are forced to spend time together as they have to plan for Jason's bar mitzvah, which he is not too excited about. Marvin and Whizzer rekindle their relationship. Through the show, Dr Charlotte grows concerned about an unknown virus that it is later heavily implied, and clear through context to be HIV/AIDS. Whizzer soon after becomes sick with the illness. Jason decides to hold his bar mitzvah at the hospital with all his friends. Unfortunately, after the party, Whizzer dies and the final scene shows his funeral. At the end of the show, Mendel sings "Welcome to Falsettoland", revealing the darker sides of the characters' lives.

Finn's Jewish upbringing inspired themes in Falsettos.

Jewish culture and identity play a significant role in Falsettos. It takes place in the "often humorous environment of Jewish neuroses and self-deprecation." Finn gave Judaism a central role in the musical, emphasized by beginning the show with the song "Four Jews in a Room Bitching". The stage version begins with all four male characters dressed in clothing from Biblical times before they remove these robes to reveal modern clothing. In the song, three characters state that they are Jewish, while Whizzer specifies that he is "half-Jewish". The first act, "March of the Falsettos", was originally intended to be titled Four Jews in a Room Bitching until Lapine insisted that Finn change the title. Writers Raymond and Zelda Knapp compared the implications of the AIDS epidemic in Falsettos to the foreshadowing of the Holocaust in the 1964 Jewish musical Fiddler on the Roof, noting that both works suggest the "comparatively innocent" atmosphere before tragedy and the "grim" environment afterward.