The observed number of galaxy clusters provides a sensitive probe of the structure of the Universe 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. the number of galaxies, X-ray luminosity, or the imprint on the Cosmic Microwave Background). I will discuss the challenges in determining mass-observable relations, and how the combination of weak gravitational lensing and X-ray observations can address these. I will review current cluster cosmology results, including those from the “Weighing the Giants” project which placed some of the tightest single-probe constraints on the dark energy equation of state. I will comment on how cluster triaxiality and orientation bias complicate our understanding of cluster selection and cluster mass proxies, especially for optically selected samples. I will conclude with an outlook towards cluster cosmology with future sky surveys, in particular the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST).