Pre-clinical research based on animal models and cell cultures allows to study normal tissue and tumour behavior. This is an important research eld for the development of new therapeutic concepts and a more in depth understanding of radiation eects on tissues. Pre-clinical research is also a highly relevant research eld in ion beam therapy to overcome the current uncertainties with respect to the radiobiological knowledge. State of the art is to perform cell culture and animal studies with X-ray units. However, the complexity of the X-ray unit varies in great extend making accurate dose calculations challenging. At the Medical University of Vienna an X-ray unit with variable energy up to 200 kV is used for irradiation of small animals and cell experiments. An identical device was recently installed at MedAustron for radiobiological experiments. Dose deposition can be controlled by individually designed lead collimators. Thus every experimental setup requires detailed individual dosimetric measurements and verication. To characterize the beam properties the half-value layers (HVL) are determined. Hence, the unpractical (and expensive) direct measurement of the X-ray spectra using spectrometers can be bypassed. Since all these measurements are very time consuming, computational methods for dose calculation, like Monte Carlo simulations, gain more and more popularity. This development goes hand in hand with the increase in computing power. One popular framework for medical applications is the simulation environment GATE (Geant4 Application for Tomographic Emissions). It evolved from a small specialized application used to simulate imaging systems, like PET and SPECT, to a more and more applicable tool for a wide variety of medical systems, including radiotherapy devices. The aim of this master project was to model the X-ray unit YXLON Maxishot with the Monte Carlo toolkit GATE and to verify the simulation outcomes with dosimetric measurements.