This chapter offers a refined overview of the concept of Research Infrastructures (RIs), clarifying their purpose, definitions, and core features across scientific domains. It synthesises the principal definitions formulated by international organisations and traces the evolution of RIs from the pioneering transnational facilities of the post-war period to the current landscape of single-sited, distributed, and virtual infrastructures. It further describes the typical life cycle of an RI, from conceptualisation and design to implementation, operation, and eventual termination, emphasising the key structural and strategic elements required at each stage. Finally, it discusses the principles guiding access and data stewardship, which underpin the provision of services and the management of research data within RIs.