Planetary rings are found around all four giant planets of our solar system. These collisional and highly flattened disks exhibit a whole wealth of physical processes involving microscopic dust grains, as well as meter-sized boulders. These processes, together with ring composition, can help to understand better the formation and evolution of proto-satellite and proto-planetary disks in the early solar system. The present chapter reviews some fundamental aspects of ring dynamics, namely their

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... attening, their stability against proper modes, their particle sizes, and their responses to resonance forcing by satellites. These concepts will be used and tested during the forthcoming exploration of the Saturn system by the Cassini mission. • Why are planetary rings so flat?