Page conjectured very recently [Phys. Rev, Lett. 71, 1291 (1993)] that if a quantum system of Hilbert space dimension mn is in a random pure state, the average entropy of a subsystem of dimension m less than or equal to n is given by Sm,n = Sigma(k=n1)(mn)1/k-m-1/2n. We outline a proof of this elegant formula.