The quantum AdS_5 x S^5 superstring at finite coupling

The integrability of string theory in AdS_5 x S^5 and of the dilatation operator of N=4 super-Yang-Mills theory has been used to propose an exact solution to the spectral problem in these theories. Weak coupling perturbation theory both in gauge theory and on the worldsheet has been extensively used to verify this solution. We discuss worldsheet methods for finding the spectrum of the AdS_5 x S^5 superstring at finite values of the coupling constant and illustrate them by recovering, within numerical errors, the predictions of the BES equation for the universal scaling function. This is the first finite-coupling calculation in this theory which uses field theory methods.