  The asteroid belt was dynamically shaped during and after planet formation.
Despite representing a broad ring of stable orbits, the belt contains less than
one one-thousandth of an Earth mass. The asteroid orbits are dynamically
excited with a wide range in eccentricity and inclination and their
compositions are diverse, with a general trend toward dry objects in the inner
belt and more water-rich objects in the outer belt. Here we review models of
the asteroid belt's origins and dynamical history. The classical view is that
the belt was born with several Earth masses in planetesimals, then strongly
depleted. However, it is possible that very few planetesimals ever formed in
the asteroid region and that the belt's story is one of implantation rather
than depletion. A number of processes may have implanted asteroids from
different regions of the Solar System, dynamically removed them, and excited
their orbits. During the gaseous disk phase these include the effects of giant
planet growth and migration and sweeping secular resonances. After the gaseous
disk phase these include scattering from resident planetary embryos, chaos in
the giant planets' orbits, the giant planet instability, and long-term
dynamical evolution. Different global models for Solar System formation imply
contrasting dynamical histories of the asteroid belt. Vesta and Ceres may have
been implanted from opposite regions of the Solar System -- Ceres from the
Jupiter-Saturn region and Vesta from the terrestrial planet region -- and could
therefore represent very different formation conditions.
