Given a noncompact manifold, a soul is a compact, totally geodesic, totally convex submanifold, such that the manifold is diffeomorphic to the normal bundle of the soul. Tuns out that every noncompact manifold with noonegative curvature has a soul, and if the manifold has positive curvature, then the soul is just a point, and the manifold is diffeomorphic to $R^n$. It was conjectured that the same statements holds for manifolds with nonnegative curvature, with positive curvature at a point. This \emph{soul conjecture} was proved in full generality by Perelmann in 1994, when he showed that the structure of noncompact manifolds with nonnegative curvature is very rigid. My goal is to go through Perelman's theorem, and show how this implies the soul conjecture.