<p>Superconducting nickelates have emerged as a new platform for exploring unconventional superconductivity and correlated-electron phenomena. Although predicted for Ni-based systems decades ago, superconductivity was only observed in 2019 in infinite-layer nickelate thin films. More recently, superconductivity with transition temperatures exceeding 80 K was discovered in Ruddlesden–Popper nickelates under high pressure. Despite these advances, both nickelate classes face major experimental challenges in synthesis and sample quality, which limits the application of various experimental techniques needed for a comprehensive understanding of the nature of their superconductivity. In this Review, we summarize recent progress in research on infinite-layer and Ruddlesden–Popper nickelates as well as their structural subcategories, outlining the theoretical foundations and key experimental findings on their electronic and magnetic properties. Additionally, we discuss the major synthesis challenges, identify open questions and future research directions and compare nickelates with other unconventional superconductors, including cuprates and iron pnictides.</p>