X-ray observations of galaxy clusters provide powerful cosmological constraints via two independent methods. The first uses measurements of the baryonic mass fraction in the largest, dynamically relaxed clusters. This method, like type Ia supernovae studies, measures distance as a function of redshift and traces the acceleration of the Universe directly. It also provides a tight constraint on the mean matter density. The second experiment uses observations of the distribution and growth of cosmic structure, as manifested in the X-ray luminosity function of galaxy clusters. It leads to tight constraints on the amplitude of mass fluctuations in the Universe, and new constraints on dark energy. I will emphasize the allowances for systematic uncertainties that must be incorporated into these experiments, and place the results in the context of other current cosmological data. I also will comment on the prospects for improving these results over the next few years.