Lecture Notes in Physics 1564 Dynamical Evolution of Viscous Discs.
Planetary rings belong to the most prominent examples of granular gases in astrophysical systems.
They are subject of permanent scientific investigation since centuries. So far, however, neither the detailled mechanisms of formation of planetary rings nor their complex spatial and dynamic behavior, originating from inelastic particle collisions, resonances, gravitational perturbations and others, are completely understood. We give a review on the present state of knowledge about planetary ring dynamics and particularly focus on the discussion of unsolved theoretical problems and unexplained observations.