  The origin of the highest energy cosmic rays is still unknown. The discovery
of their sources will reveal the workings of the most energetic astrophysical
accelerators in the universe. Current observations show a spectrum consistent
with an origin in extragalactic astrophysical sources. Candidate sources range
from the birth of compact objects to explosions related to gamma-ray bursts or
to events in active galaxies. We discuss the main effects of propagation from
cosmologically distant sources including interactions with cosmic background
radiation and magnetic fields. We examine possible acceleration mechanisms
leading to a survey of candidate sources and their signatures. New questions
arise from an observed hint of sky anisotropies and an unexpected evolution of
composition indicators. Future observations may reach the necessary sensitivity
to achieve charged particle astronomy and to observe ultrahigh energy photons
and neutrinos, which will further illuminate the workings of the universe at
these extreme energies. In addition to fostering a new understanding of
high-energy astrophysical phenomena, the study of ultrahigh energy cosmic rays
can constrain the structure of the Galactic and extragalactic magnetic fields
as well as probe particle interactions at energies orders of magnitude higher
than achieved in terrestrial accelerators.
