This chapter provides a practical introduction to the legal environment governing European Research Infrastructures (RIs), aimed at readers with predominantly scientific backgrounds. It outlines the core features of the national, European, and international normative systems that shape the legal and organizational structures of RIs, highlighting how these frameworks influence strategic decisions. Through two empirical cases, the chapter illustrates the significant impact of public procurement regimes on the choice of legal model, clarifying when motivations—such as VAT advantages or exemptions from EU procurement rules—are appropriate or misguided. The discussion then shifts to the internal legal tools that RIs require for their daily operations, including contracts that regulate access, employment, data protection, intellectual property, and ethical considerations. The international dimension of RIs adds further complexity by raising questions about applicable law and jurisdiction in cross-border contractual relationships.