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I will briefly discuss the occurrence and detection of spiral density waves and the viscous overstability in Saturn's rings. The former typically being excited by moons orbiting outside the rings, with typical wavelengths of some tens of kilometers, the latter being a spontaneous instability of the ring flow, instigating axisymmetric density wave trains on a 100m scale. The study of these wave structures can provide insights in principles of disk-satellite interaction and disk dynamics, and is therefore of fundamental interest for astrophysical disks. For instance, the actions of moons that orbit interior and exterior to Saturn’s rings bear many similar- ities to those of planets forming in proto-planetary disks. Furthermore, by utilizing a fluid description of the rings, the analysis of these wave structures in Saturn’s rings can provide important physical pa- rameters which are otherwise hard to constrain, such as the local mass surface density and the viscosity of the rings.