  This chapter of the book Planetary Ring Systems addresses the origin of
planetary rings, one of the least understood processes related to planet
formation and evolution. Whereas rings seem ubiquitous around giant planets,
their great diversity of mass, structure and composition is a challenge for any
formation scenario. Recent advances in our understanding of ring and satellite
formation and destruction suggest that these processes are interconnected, so
that rings and satellites may be two aspects of the same geological system.
However, no single theory seems able to explain the origin of the different
planetary rings known in our Solar System, and it now seems evident that rings
may result from a variety of processes like giant collisions, tidal stripping
of comets or satellites, as well as planet formation itself.
  In order to build any theory of ring formation it is important to specify
physical processes that affect the long-term evolution of rings, as well as to
describe the different observations that any ring formation model should
explain. This is the topic of section 2. In section 3, we focus our attention
on Saturn's rings and their main properties, and then discuss the pros and cons
of a series of ring formation models. We also discuss the link between rings
and satellites. In section 4, we extend the discussion to the other giant
planets (Jupiter, Uranus, and Neptune). Section 5 is devoted to new types of
rings -- the recent discovery of rings orbiting small outer Solar System bodies
(Centaurs), and the possible rings around extrasolar planets. In section 6, we
conclude and try to identify critical observations and theoretical advances
needed to better understand the origin of rings and their significance in the
global evolution of planets.
