Following the dynamics of the Universe with galaxy cluster counts

Number density of galaxy clusters as a function of mass and redshift can be used to constrain Omega_M, Omega_Lambda and, more weakly, the dark energy equation of state. This technique exploits the remarkable sensitivity of the cluster counts to the growth of linear density perturbations. It also uses the cosmological distance-redshift and volume-redshift dependencies which are built into observational determinations of the cluster mass function at z>0. The cluster counts test was proposed in different forms years and decades ago. It has since been implemented using the data of ever improving quality. I will present the latest results based on Chandra observations of massive clusters discovered in the ROSAT surveys.