Abstract: First we present a brief overview of the contemporary state of the High-energy Physics, with the emphasis put on the validation of the Standard Model. We will mention searches for Higgs boson(s), several production and detection mechanisms, and make a quick overview of the theories Beyond the Standard Model. Then we will describe the todays most promising particle collider LHC and mainly the state-of-the-art detector ATLAS and its subsystems; the special focus will be on the Pixel detector. Next we mention the ATLAS offline computing. The principles and use of the Athena - ATLAS offline software framework will be discussed. Finally the speaker's own research activity will be described: the effect of cosmic rays on the Pixel detector. This is done using Geant4 simulation within Athena. All the software tools will be introduced and described, the preliminary results will be then used to discuss their efficiency. Subsequently, the cosmic rate and the distribution of hits and energy will be estimated. At the closing, we will present an outlook how to improve the cosmic rate estimate by folding in the trigger efficiency simulation.