ray energy spectrum has been measured over almost 13 decades in energy, roughly between $\sim 0.1$GeV and $10^{12}~$GeV ( = 1 ZeV). At lower energies the solar wind traps extra-solar-system charged particles and carries them away. At higher energies the extrapolated fluxes are extremely small at best ($< 10$ particles km$^{-2}$ century$^{-1}$), so not yet measured or adequately constrained. To a first approximation the measured energy spectrum is a broken powerlaw with a slope close to -2.7 below a “knee” near $10^6$GeV, then steepening by about 0.5 in the index above the knee. The spectrum appears to flatten slightly again above $\sim 10^9$GeV (= 1 EeV). This feature is sometimes called the “ankle” of the CR spectrum. The knee and ankle features are commonly viewed as corresponding to transitions either in the nature of the CR accelerator or in the propagation of the CRs. As outlined below there is currently a controversy about the spectrum above 100 EeV (Abu-Zayyad, 2002; Takeda, 2002), but several groups have independently detected events with energies in this range, so there is little doubt about their existence. The highest energy event reported so far is 0.3 ZeV (Bird, 1994). That amounts to an astounding 50 