Joules of kinetic energy in an individual elementary particle. Not only is it problematic to explain the generation of CRs at these high energies, their propagation is severely limited, especially by interactions with the cosmic microwave background (CMB). The difficulties and implications are obviously very important, so there are major efforts underway to improve on current measurements.

The composition of the cosmic rays near or above the knee is not well established. As discussed by Stecker in his contribution at this School (Stecker 2002) the evidence mostly supports a transition from a mix of light and heavy nuclei near and just above the knee to mostly light nuclei at the highest energies. Most, but not all discussions of UHECRs take them to be protons for that reason. Additionally, collisions between ultrahigh energy heavy ions and photons disrupt heavy ions, so that they cannot propagate over long range. (See Stecker 2002 for a discussion of these properties.)

Another important property of the UHECRs that constrains their origins or possibly their propagation is the distribution of detected events on the sky. Below the knee CR detections are nearly isotropic (e.g., Erlykin, 2002), but that is a reflection of that fact that low 