In 2003, Ozsvath and Szabo defined a knot invariant, \tau, using the knot filtration on the Heegaard Floer complex, and showed that \tau gives a lower bound on the 4-ball genus of the knot. In this talk, I will define \tau, state some of its properties, and use these properties to prove that the four ball genus of the (p, q) torus knot is (p-1)(q-1)/2 (a conjecture of Milnor, originally proved in 1993 by Kronheimer and Mrowka using Donaldson's invariants).