The research project which this website, report and database are coming out of, is based at the University of Amsterdam, Department of Sociology, and is financed by a grant of the Dutch Research Council. The project started in 2020 and is expected to be completed in 2025. It is the first to study the rise of private museums globally in a systematic way. The rise of private museums, which has been particularly rapid in the 21st century, is highly controversial: according to some they are neo-aristocratic institutions which translate economic into cultural inequality, while others argue that they democratize and support art, especially when government support is absent or declining. Given these controversies, it is striking how little we actually know about them. The project seeks to fill this knowledge gap by studying what motivates collectors to found a private museum; how these museums position themselves in art worlds through their acquisitions and exhibition programs; and what the impact of private museums is on these art worlds. The project has both a qualitative and a quantitative component: it involves among others ethnographic fieldwork in Brazil, in-depth interviews in Germany and Turkey, the joint development of the database which this website makes accessible, and statistical analysis aimed at understanding if, how and when the careers of artists or the value of art is impacted in case private museums take an interest.