Essay on the Poincaré Conjecture

I wrote this essay in 2007 for publication in an Italian encyclopedia of mathematical problems. At this time it was still not 100% certain that Perelman's proof of the 3-dimensional conjecture was correct and this is why I have been a little tentative about his proof. If writing now I would change the title of section 7 from "Perelman's program" to "Perelman's proof". Nothing else needs updating.

Essay on the Poincaré conjecture