A correct energy calibration for jets is essential to the success of the ATLAS experiment. In this thesis I study a method for deriving an in situ jet energy calibration for the ATLAS detector. In particular, I show the applicability of the missing transverse energy projection fraction method. This method is shown to set the correct mean energy for jets. Pileup effects due to the high luminosities at ATLAS are also studied. I study the correlations in lateral distributions of pileup energy, as well as the luminosity dependance of the in situ calibration method.