Musical theatre is a form of theatre combining songs, spoken dialogue and dance. The emotional content of the piece – humor, pathos, love, anger – as well as the story itself, is communicated through the words, music, movement and technical aspects of the entertainment as an integrated whole. Although musical theatre overlaps with other theatrical forms such as opera, it may be distinguished by the equal importance given to the music as compared with the dialogue, movement and other elements of the works. Since the early 20th century, musical theatre stage works have generally been called, simply, "musicals".

Musicals are performed all around the world. They may be presented in large venues, such as big budget West End and Broadway theatre productions in London and New York City, or in smaller fringe theatre, Off-Broadway or regional theatre productions, on tour, or by amateur groups in schools, theatres and other performance spaces. In addition to Britain and North America, there are vibrant musical theatre scenes in many countries in Europe, Latin America, Australasia and Asia.

Although music has been a part of dramatic presentations since ancient times, modern musical theatre emerged during the 19th century, culminating with the works of Gilbert and Sullivan in Britain and those of Harrigan and Hart in America, followed by the numerous Edwardian musical comedies and the works of American creators like George M. Cohan. Early in the 20th century, the Princess Theatre musicals and other smart shows like Of Thee I Sing were artistic steps forward beyond revues and other frothy entertainments and led to such groundbreaking works as Show Boat and Oklahoma!. Some of the most famous and iconic musicals through the decades that followed include West Side Story, The Fantasticks, Hair, A Chorus Line, Les Misérables, The Phantom of the Opera, Rent, The Producers and Wicked.