The importance of very massive clusters of galaxies has long been recognized for applications ranging from cosmology to studies of the distant universe through gravitational lensing. Until recently the rarity of these extreme objects has, however, limited such studies to the local universe. We here report on MACS, the first survey to provide a statistical sample of massive, X-ray luminous galaxy clusters at redshifts z > 0.3. This presentation is aimed at generalists and specialists alike. The survey, the resulting cluster sample, and the relevance of X-ray selection will be discussed, and results will be presented ranging from measurements of cosmological parameters, through a characterisation of large-scale structure at z > 0.5, to an assessment of the impact of environment on galactic star formation.