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Abstract: The ATLAS trigger system is based on three levels of event selection that selects the physics of interest from an initial bunch crossing rate of 40MHz to an output rate of compatible with the offline computing power and storage capacity. During nominal LHC operations at a luminosity of , decisions must be taken every 25ns. The LHC is expected to begin operations with a peak luminosity of with far fewer number of bunches, but quickly ramp up to higher luminosities. Hence, the ATLAS Trigger and Data Acquisition system needs to adapt to the changing beam conditions preserving the interesting physics and detector requirements that may vary with these conditions. [Copyright &y& Elsevier]