  This chapter explores the physics shared by planetary rings and the various
disks that populate the Universe. It begins with an observational overview,
ranging from protoplanetary disks to spiral galaxies, and then compares and
contrasts these astrophysical disks with the rings of the Solar System.
Emphasis is placed on fundamental physics and dynamics, and how research into
the two classes of object connects. Topics covered include disk formation,
accretion, collisional processes, waves, instabilities, and satellite-disk
interactions.
