Abstract: Unlike simple liquids, polymers usually solidify via the glass transition – a process in which the viscosity of the liquid increases by orders of magnitude as the temperature is lowered below the glass transition temperature (Tg), albeit the molecular arrangement hardly changes. Recently, numerous studies show that the Tg of polymer films can change notably when the film thickness is decreased to the nanometer range. This implies that the viscosity of the films can change significantly as well. In this talk, I shall discuss the viscosity measurement we carried out on two polymer film systems - one in which the Tg decreases with decreasing film thickness, and one in which the Tg increases. Our data reveal that the viscosity of the nanometer films changes by orders of magnitude, concomitant with the Tg. Importantly, we uncover that the viscosity can be fully described by presuming the films to be a bi-layer, comprising a bulklike layer, hydrodynamically coupled to a nanometer thick, novel interfacial layer. For the system where the Tg decreases, the interfacial layer is mobile and located at the free surface. For the system where the Tg increases, the interfacial layer is slow and located at the substrate surface. These results point to the importance of the interfaces on the dynamic properties of nano-confined polymers.