  The High Resolution Fly's Eye (HiRes) experiment has observed the GZK cutoff.
HiRes observes two features in the ultra-high energy cosmic ray (UHECR) flux
spectrum: the Ankle at an energy of $4\times10^{18}$ eV and a high energy
suppression at $6\times10^{19}$ eV. The later feature is at exactly the right
energy for the GZK cutoff according to the $E_{1/2}$ criterion. HiRes cannot
claim to observe a third feature at lower energies, the Second Knee. The HiRes
monocular spectra are presented, along with data demonstrating our control and
understanding of systematic uncertainties affecting the energy and flux
measurements.
