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Résumé : The discovery of ring systems around Uranus and Jupiter, and the Pioneer and \nVoyager spacecraft observations of Saturn, have shown that planetary rings \nare both more common and more complex than previously suspected. These \nring systems, interesting in their own right, also serve as prototypes for more \nmassive disk systems such as accretion disks and spiral galaxies occurring \nelsewhere in astronomy. \nDisks and rings are a natural consequence of dissipation in rotating systems. \nA cloud of debris surrounding a spherical planet settles into a flat circular ring \nbecause interparticle collisions dissipate energy but conserve total angular \nmomentum. Since planets are oblate, only the component of angular momentum \nalong the spin axis is conserved, and the flat ring lies in the equatorial \nplane. \nCollisions redistribute angular momentum among the particles and the ring \nspreads, transferring mass inward and angular momentum outward (Lynden-Bell \n& Pringle 1974). However, the spreading process occurs on a much \nlonger timescale than the flattening process since the collision speeds in a flat \nring are much lower than the orbital speeds (see Sections 2.2 and 5. 3). \nSpreading can be slowed by gravitational interactions with satellites; nevertheless, \na ring cannot live forever and an important constraint on possible ring models is that they yield survival times at least comparable to the age of the \nsolar system (cf. Sections 5, 6). \nThis review was written in October 1981, shortly after the Voyager 2 \nencounter with Saturn. Analysis of the data from the Voyager encounters is not \nyet complete. Therefore the emphasis in this review is on the basic physical \nprocesses that occur in planetary rings, rather than on a detailed confrontation \nof theoretical predictions with observation. \nTable 1 lists some of the important properties of the planets with known ring \nsystems. \nFor the sake of brevity we shall refer to a series of papers we have written \non rings (Goldreich & Tremaine 1978a, b, c, 1979a, b, c, 1980, 1981) as GT \n1, . . . , GT 8.