The discovery of high-temperature superconductivity in the bilayer nickelate La₃Ni₂O₇—both in bulk form under high pressure and in thin films at ambient pressure—and related Ruddlesden-Popper nickelates have generated considerable interest in understanding the underlying mechanisms. In this study, we address the effects of orbital-selective electron correlations —where some electronic orbital states experience stronger renormalization effects than others. We show how such effects influence the low-energy electronic structure [1,2] and electronic dynamics [2], and how they drive the superconducting pairing through both interlayer and intralayer exchange interaction pathways [3]. Our theoretical findings, especially on the orbital-selective effects, compare well with experimental results, including spectroscopic measurements that shed light on the underlying correlation effects in the normal state as well as the measurements on the pairing gap structure in thin films of the bilayer system at ambient conditions. For the latter, we especially highlight the role of orbital selectivity in determining the angular variation of the superconducting gap and the two-peak structure of the tunneling spectrum. We discuss the implications of these findings for the understanding of both pressurized bulk La₃Ni₂O₇ and its strain-engineered thin-film counterpart. Trilayer and other multilayer nickelates will also be discussed [4].