We present results on the ultra-high energy cosmic ray spectrum as measued by the High Resolution Fly's Eye Experiment. The spectrum is analyzed in two different ways: monocular reconstruction and stereo reconstruction. The monocular spectrum has the highest statistics while the stereo spectrum has the best resolution.The monocular spectrum has a threshold of .1 EeV while the stereo spectrum's energy threshold is near 1 EeV. Both spectra show strong evidence of structure, including the ankle, near 3-5 EeV and the GZK cutoff near 60 EeV. In particular, the GZK cutoff has been clearly observed at the greater than five sigma level.

High Resolution Fly's Eye (HiRes)

Proceedings of the 30th International Cosmic Ray Conference; Rogelio Caballero, Juan Carlos D'Olivo, Gustavo Medina-Tanco, Lukas Nellen, Federico A. Sánchez, José F. Valdés-Galicia (eds.); Universidad Nacional Autónoma de México, Mexico City, Mexico, 2008; Vol. 4 (HE part 1), pages 473-476