Von Neumann algebras are weakly closed *-algebras of bounded operators on a Hilbert space. They naturally arise in the context of representation theory, ergodic theory and measurable group theory. Von Neumann algebras come in two very different flavors, distinguished by the key notion of amenability. After a basic introduction to von Neumann algebras, I will give an overview of Alain Connes' classification of amenable factors and then present the groundbreaking work of Amine Marrakchi on the structure of nonamenable factors of type III.