Is there maximal mixing in the lepton sector?

We discuss the potential of long-baseline neutrino oscillation experiments to determine deviations from maximal ${\nu }_{\mu }\mathrm{\text{−}}{\nu }_{\tau }$ mixing. We compare the obtainable sensitivities to predictions from neutrino mass models and to the size of quantum corrections. We find that the theoretical expectations for deviations are typically well within experimental reach.