As the most massive objects in the Universe, and likely the end point in the hierarchical structure formation, galaxy clusters offer unique insight into cosmological growth of structure. Several observational campaigns, including X-ray, optical and microwave telescopes have either just started to collect data on clusters, or will start in the near future. From the theory side, there are two major road blocks for doing cluster cosmology: predicting abundance of clusters for different cosmological models, and finding precise way to measure cluster masses from a given observable. In this talk, I will present our latest results on the mass function from numerical simulations, outlying some future challenges. I will also present a method for classifying clusters as "relaxed" or "merging" in observationally applicable way, and how their fraction can be used for cluster cosmology. I will also show how this classification improves the quality of mass-observable relations. Finally, throughout this talk, I will showcase application of cluster cosmology on two kinds of cluster surveys (X-ray and SZ) and putting constrains on early dark energy models.