Surveys of galaxy clusters provide a sensitive probe of cosmology by measuring the evolution of the halo mass function. However, already current cluster surveys are systematically limited by uncertainties in the relation between cluster mass and observables (e.g. X-ray luminosity, cluster richness). Cluster weak lensing is the most promising observational method to calibrate the mass scaling to the required precision, but requires the control of systematic errors to a few percent each. In the "Weighing the Giants" project, we carefully investigated and quantified all sources of systematic uncertainty, resulting in accurate weak lensing masses for 51 clusters. We use these measurements to improve the precision of cosmological constraints from X-ray selected clusters by a factor of two. Already from a sample of ~200 clusters selected from the ROSAT All-Sky Survey, we place some of the tightest, most robust constraints on a number of cosmological parameters, including the dark energy equation of state, neutrino masses, and modified gravity. Furthermore, we show that when adopting the "Weighing the Giants" mass scale, the results from Planck CMB temperature anisotropies and Planck cluster counts are consistent without invoking the need for new physics. These results bode extremely well for future cluster surveys. In particular, I will show how the "Weighing the Giants" work lays out the path for LSST to become a key cornerstone for cluster experiments in the next decade.
