Viscosity values as estimated from isostatic processes and from temperature and melting point-depth relations show that the Moon's outer layers are much more viscous than any geologic region on Earth. These high $\eta $-values are responsible for the survival of the very old lunar landscape. At depths below 500 km $\eta $-values increase and permit a limited convection. Moonquakes around 800 km depth seem to be connected with a limited range of viscosity values. A tentative relation between viscosity and seismic Q-values is presented.