I will discuss the main results of a series of papers in which we derive simultaneous constraints on cosmological and X-ray scaling relation parameters using observations of the growth of massive, X-ray flux-selected galaxy clusters. Our data set consists of 238 cluster detections from the ROSAT All-Sky Survey, and incorporates follow-up observations of 94 of those clusters using the Chandra X-ray Observatory or ROSAT. We have implemented a new statistical framework to self-consistently produce simultaneous constraints on cosmology and scaling relations from such data, accounting for survey biases, parameter degeneracies and the impact of systematic uncertainties. I will present tight constraints on models of dark energy, and on the luminosity-mass and temperature-mass relations, which, as I will discuss, lead to important results on cluster astrophysics. I will also present improved constraints on departures from General Relativity (GR) on cosmological scales, using the growth index, gamma, to parameterize the linear growth rate of cosmic structure. Combining the X-ray cluster growth data with cluster gas-mass fraction, SNIa, BAO and CMB data we find a tight correlation between gamma and sigma_8. Allowing w to take any constant value, we simultaneously constrain the growth and expansion histories, and find no evidence for departures from either GR or the LCDM paradigm. We also obtain robust constraints on neutrino properties, such as the species-summed neutrino mass and the effective number of neutrino species, that are enabled by the precise and robust constraint on sigma_8 from our data. Our results highlight the power of X-ray studies, which enable the straightforward production of large, complete, and pure cluster samples and admit tight scaling relations, to constrain cosmology. However, the new statistical framework we apply to this task is equally applicable to cluster studies at other wavelengths.