Music and movies are inseparable. Even during the era of silent movies there would often be musical accompaniment from a piano player or an orchestra in more extravagant theaters. Tone, pacing, and atmosphere are all impacted by the music used in a film. Musicals exploit the intertwined relationship of film and music by having characters literally sing their emotions to the audience, in the world of the movie musical it is entirely natural for you to break out into song and it would be impolite for complete strangers not to join you for your densely choreographed dance number. Movies that aren't musicals obviously have different rules; characters don't break into song all willy-nilly, there has to be a reason for such tom-foolery (both in the narrative sense and in order to deal with the film's thematic concerns). This list deals only with diegetic music, meaning that it exists in the world of the characters (rather than a song on a soundtrack that the characters can't hear), and that it is music actually being played or sung by characters in the film. This means that songs that simply play over the scene don't count (sorry Mean Streets) and songs that characters hear on the radio, even if they react to it, don't count either (sorry Reservoir Dogs). Some seemingly obvious choices have been omitted (I don't need to hear Sam play it again), but every film on this list uses musical asides in a meaningful and iconic way.