  In this chapter I give an overview of shock acceleration, including a
discussion of the maximum energies possible and the shape of the spectrum near
cut-off, interactions of high energy cosmic rays with, and propagation through,
the background radiation, and the resulting electron-photon cascade. Possible
sources of the highest energy cosmic rays are discussed including active
galaxies, gamma ray bursts and topological defects. I argue that while the
origin of the highest energy cosmic rays is still uncertain, it is not
necessary to invoke exotic models such as emission by topological defects to
explain the existing data. It seems likely that shock acceleration at
Fanaroff-Riley Class II radio galaxies can account for the existing data.
However, new cosmic ray data, as well as better estimates of the extragalactic
radiation fields and magnetic fields will be necessary before we will be
certain of the origin of the highest energy particles occurring in nature.
