Cosmic rays with energies above 1 EeV are currently of considerable interest in astrophysics and are to be further studied in a number of projects which are either currently under construction or the subject of well-developed proposals. This paper aims to discuss some of the physics of such particles in terms of current knowledge and information from particle astrophysics at other energies. Included is an argument that the role of galactic sources at these ultra high energies should not be ignored. Also, the key role of the unknown structure and magnitude of the intergalactic magnetic field is emphasised.