The ATLAS detector is an all-purpose detector to study high-energy proton–proton collisions. ATLAS is located at the LHC (Large Hadron Collider) at CERN in Geneva, Switzerland. Before first data taking, many beam tests have been carried out in order to fully understand each detector component. The studies in this thesis will concentrate on the 2004 beam test of the entire combined end-cap calorimeter system. The first section of this thesis outlines particle selection in the incoming test beam, eliminating contamination in order to have an accurate calibration environment. The remainder of the thesis focuses on detector calibration and performance studies, including signal-to-energy calibration constant determination, and various detector energy summation methods studying their effect on response. Overall detector energy sharing characteristics including the response of dead detector regions is also presented.