Clusters of galaxies provide us with crucial information for the understanding of the cosmology of our Universe. Observations are becoming increasingly precise and simulations improving their accuracy, challenging theories. We have analysed how the new Millennium Gas Simulation concurs with theory and observation. We first introduce the basic principles of the $\Lambda$CDM cosmological model, including the FRW metric. We then explain how structure formation leads to galaxy and cluster formation. Clusters are the most massive gravitationally stable objects in the Universe; we describe their typical properties and key observables and how the Intracluster medium (a very hot, low density, optically thin, plasma) X-ray emission informs us about the characteristics of these objects. The use and necessity of cosmological simulations is also discussed. We give an overview of the dark matter Millennium Simulation and the Millennium Gas Simulations which also include baryonic matter to the equation.We briefly go through every step necessary to produce these simulations and show simulated X-ray surface brightness maps eventually produced. We study the catalogues associated to each cluster, then we test the ability of the simulation to reproduce the expected mass function and scaling relations of clusters of galaxies. Our main results focus on the surface brightness profiles of clusters; we define this property as well as the fitting model known as $\beta$ model. A series of case studies leads us to discuss whether the key parameters are correlated with one another and what they can tell us about the clusters. We establish that working with a large number of clusters will soften the influence of the radial range we are using to fit the $\beta$ model. Finally we compare the simulated sample to three recent observational data sets, and show that they are in reasonable agreement.