ABSTRACT: Galaxy clusters are dark-matter dominated systems enclosed in a volume that is a high-density microcosm of the rest of the universe. How universal are their thermodynamical profiles? What is the effect of any non-thermal pressure support and how can we quantify it? Does the assumption of spherical symmetry hold? What is their true mass scale? I will describe the lessons that we have learnt on these fundamental questions with our current projects, such as the XMM-Newton Cluster Outskirts Project (X-COP) and CLASH. More has to be understood and will be the focus of our next XMM-Newton Heritage Cluster Project that will pave the way in using the next generation of X-ray observatories, like eROSITA, XRISM and Athena, to construct a consistent picture of the formation and composition of galaxy clusters.