Using measurements of the abundance of galaxy clusters we
obtain constraints on dark energy and gravity at
cosmological scales. Our data set consists of 238 cluster
detections drawn from the ROSAT All-Sky Survey and X-ray
follow-up observations of 94 of those clusters. Using a new
statistical framework we self-consistently and
simultaneously constrain cosmology and observable-mass
scaling relations accounting for survey biases, parameter
covariances and systematic uncertainties. Allowing the
linear growth index and the dark energy equation of state to
take any constant values, we find no evidence for departures
from GR+LCDM. If time permits, I will also present
preliminary results on testing an alternative gravity model
using our cluster data sets. Our results highlight the power
of X-ray cluster studies to constrain cosmology