I discuss the role that X-ray galaxy clusters plays in the determination of cosmological parameter. In particular I focus the discussion on the evolution of the cluster mass function for the determination of the density parameter Ωm and the normalization of the power spectrum σ8. Available constraints indicate that Ωm lies in the range 0.2–0.5, with σ8 − 0.7–0.8 for Ωm=0.3. I emphasize that the ability of clusters to provide such constraints is determined by the possibility of relating X-ray observable quantities to the cluster mass and, therefore, to our understanding of the physical processes which determine the properties of the ICM. I finally discuss the relevance that hydrodynamical simulations of clusters can play to understand the ICM physics and to calibrate mass estimates from X-ray observational quantities.