Measurement of the energy spectrum of cosmic rays is one of the key results of the Pierre Auger Observatory. Five different methods currently used at the Observatory to estimate the energy spectrum are presented. The main focus is given to the low-energy extension of the measurement utilizing for the first time Cherenkov-dominated data collected by the fluorescence detector. The energy spectrum combined from available methods starts at 10^15.5 eV and spans over more then four decades in energy allowing us to observe four spectral features connected to changes in the primary cosmic ray beam. Ongoing upgrade of the Pierre Auger Observatory and comparisons with other experiments are discussed.
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