Considerable effort in cosmology is focused on understanding the statistical nature of the cosmological density field that underlies the observable large-scale structure. Clusters of galaxies trace the highest, non-Gaussian peaks of the matter density, and their abundance is a sensitive probe of both underlying cosmological parameters as well as of the physics governing structure formation. In my talk I will explain how cluster abundance can be used to search for modifications of gravity via the impact on spherical collapse and why neutrinos are a crucial component in these searches. In addition, astrophysical signatures within the cluster can be a key signal themselves and I will demonstrate how to incorporate them consistently in searches to constrain deviations from general relativity.