  Understanding the acceleration of Ultra High Energy Cosmic Rays is one of the
great challenges of contemporary astrophysics. In this short review, we
summarize the general observational constraints on their composition, spectrum
and isotropy which indicate that nuclei heavier than single protons dominate
their spectra above $\sim 5\,{\rm EeV}$, that they are strongly suppressed
above energies $\sim50\,{\rm EeV}$, and that the only significant departure
from isotropy is a dipole. Constraints based upon photopion and
photodisintegration losses allow their ranges and luminosity density to be
estimated. Three general classes of source model are discussed - magnetospheric
models (including neutron stars and black holes), jet models (including Gamma
Ray Bursts, Active Galactic Nuclei and Tidal Disruption Events) and Diffusive
Shock Acceleration models (involving large accretion shocks around rich
clusters of galaxies). The value of constructing larger and more capable arrays
to measure individual masses at the highest energies and probably identifying
their sources is emphasized.
