Open educational resources (OER) have generated a considerable amount of attention in recent years in the field of open, distance, and digital education (ODDE). Digital knowledge infrastructures of different kinds have enabled the creation, storage, management, sharing, and adoption of these resources across educational sectors, levels, and geographies. This chapter presents a general overview of these infrastructures, the underpinning models of OER provision, main characteristics, and key insights from research. It draws on the literature and discusses examples purposively selected to illustrate the diversity of scope, educational stages, and types of online OER infrastructures established on a global, national, regional, or institutional scale. Key challenges are also discussed, including licensing issues, concerns about quality assurance, metadata problems, the sustainability of the initiatives, and sociocultural aspects, among others. In addition to revisiting the conception and adoption of OER in different cultures, important topics to be further addressed by future ODDE research are presented.