The implementation and impact of international large-scale learning assessments (ILSAs) has grown significantly over the past two decades. The aim of this paper is to identify and understand the reasons for participation and non-participation, in order to inform debates about the costs and benefits of ILSAs for participating countries, international organizations and the international community more broadly. Participation can be understood from multiple perspectives, with different implications for how we conceive appropriate use and misuse, and thus for evaluating the benefits of participation in relation to the costs. The paper provides a short history of the ILSA phenomenon, before reviewing different rationales that governments have for participating or not participating in ILSAs, organised into a four-dimensional framework: political, economic, technical and socio-cultural. We argue that debates about whether ILSA participation is ‘worth it’ must take account of the diverse purposes of participants in these assessments.