Recent Sunyaev–Zeldovich (SZ) surveys (ACT, Planck, SPT) have provided new cluster catalogs, very significantly expanding the coverage of the mass–redshift plane, while XMM‐Newton surveys are pushing cluster detection to lower masses, up to z ∼ 1 and beyond. Cosmological analysis of the new cluster samples, particularly that of the largest Planck sample, found fewer clusters than predicted by the base Planck ΛCDM model derived from cosmic microwave (CMB) anisotropies. This could imply a need for extension of the model and/or a revision of cluster physics. In parallel, Chandra and XMM‐Newton follow‐up programs on these new SZ‐discovered clusters have improved our knowledge of evolution, as well as providing new information on survey selection. These results challenge our understanding of fundamental issues such as (a) what the true mass of clusters is, with implications for the baryon depletion at cluster scales and cluster dynamical evolution, and (b) what the true underlying population is, that we are only partly detecting at various wavelengths. Potential new observations to address these issues with XMM‐Newton in conjunction with other observatories are discussed.