The main objective of this thesis is the measurement of the cosmic ray energy spectrum above \unit[1]{EeV} based on the data recorded at the Pierre Auger Observatory. The continuation of the spectrum in the form of a power law is rejected with a 5.2 sigma significance. The number of events expected if a power law holds, above 40 EeV are 167+-3 and 35+- 1 above 100 EeV whereas in the measured flux there are 64 events and 1 event