The abundance of massive halos (and of the galaxy clusters they host) has long been recognized as an extremely promising probe of the large-scale structure of the universe. Over the past decade, tremendous progress was made, notably thanks to the availability of high-resolution surveys of the Cosmic Microwave Background (CMB), of high-quality measurements of gravitational lensing, and of advanced numerical simulations.

The sample of galaxy clusters selected by the South Pole Telescope (SPT) in the CMB now exceeds a thousand objects. The Dark Energy Survey (DES) allows for measurements of gravitational lensing for almost 700 sample clusters with exquisit control over systematic uncertainties. We supplement this dataset with 39 lensing measurements of high-redshift clusters with the Hubble Space Telescope (HST). The joint analysis of the cluster abundance and weak-lensing mass calibration provides tight cosmological constraints that are competitive with other major probes.

In my talk, I will review the SPT cluster cosmology and mass calibration program. I will focus on the latest SPT + DES Y3 + HST analysis and present new cosmological constraints.