I'm not a big fan of Dario Argento but it had to said that this is one of the very best horror films. Set in a dance academy, of all places, it immediately unsettles you even before you learn the true purpose of this establishment. You're never quite sure whether the whole thing is an extended dream of the protagonist, Suzy. The film veers dreamily between the baroque sets with lurid colours, and stark reality.
The cinemaography and set design contribute greatly to the tension as does the music provided by Argento's own rock band The Goblins. A remake was recently touted but mercifully the idea has now been scrapped because the would-be makers couldn't see anything inherently evil in a dance academy.