Wise's malnormal special quotient theorem (MSQT) is a key ingredient in the recent resolution by Agol of some central conjectures in 3-manifolds, including the virtual Haken conjecture. The MSQT allows one to perform certain small-cancellation operations or "Dehn fillings" on a virtually special hyperbolic group, in such a way that the result is still hyperbolic and virtually special. I'll try to give an idea how this theorem is used and outline a new proof by Agol, Groves, and myself.