Harald Ebeling, IfA The most massive clusters of galaxies: astrophysical and cosmological applications Containing over 10^15 solar masses of dark matter, X-ray emitting gas, and galaxies, clusters are giant laboratories for studies of the interaction and evolution of matter on cosmological timescales. Focusing on X-ray selected samples of the rarest, most massive clusters, I will review the scientific rationale behind 10-year projects like MACS, and present an overview of a wide range of results obtained by both in-depth studies of individual systems and statistical analyses of all-sky samples. Specifically, I will touch upon our findings on the physical mechanisms driving galaxy evolution as a function of environment, gravitational lensing, and a tantalizing possibility of using the kinematic Sunyaev-Zeldovich effect to probe the fabric of the pre-inflationary Universe.