The European Space Agency (ESA) is an international intergovernmental organization looking back on half a century of achievements in exploring and using outer space. The legal foundation of the Agency is an international treaty commonly known as the ESA Convention, which was opened for signature on 30 May 1975. Since then, with 11 founding States, ESA has doubled its European membership. With its twenty-two member States, three associate members and many international cooperation partners, ESA shapes spaceflight, accelerates European space industry and fosters international partnerships in one of the most exciting domains of human endeavour. The Convention is the fundament of ESA’s legal framework, or – using a different picture – the top of its ‘legal pyramid’, establishing the ESA legal framework for governmental programmes and partnering activities. The Convention is a powerful tool for cooperation , but doesn’t grant ESA any direct regulatory power over its member states. ESA secondary law – such as Programmes, Financial rules, Procurement regulations, administrative instructions – are strongly derived from its mandate. The Agency’s purpose, in the words of its Convention, shall be to provide for and to promote for exclusively peaceful purposes, co-operation among European States in space research and technology and their space applications, with a view to their being used for scientific purposes and for operational space applications systems. This shall be achieved through elaborating and implementing a long-term European space policy, elaborating and implementing space activities, co-ordinating the European space programme and national programmes, and by elaborating and implementing an appropriate industrial policy. ESA has two organs: the Council – the Agency’s plenary body composed of representatives of its member states – and the Director General supported by staff, representing the executive side of ESA. Together, they allow ESA to act as an institution with full legal capacity in all jurisdictions. Emanating as a first layer under the Convention are the Agency’s rules and regulations adopted by Council, which apply to the Agency’s own programs. Those include, inter alia, the Financial, Staff, Security and Procurement Regulations, as well as General Clauses and Conditions for ESA contracts, Rules on Information, Data and Intellectual Property and Rules on Arbitration, which can be found here: https://www.esa.int/About_Us/Law_at_ESA/Highlights_of_ESA_rules_and_regulations. ESA’s optional programmes are based on Programme Declarations, which are international agreements concluded among all or some of ESA’s member States. Annex III of the Convention defines legal conditions for the formation, execution and termination of these programmes. ESA also concludes international agreements with other international organisations and institutions, and with governments, space organisations and institutions of non-member states. Such agreements form the basis of international cooperative space programmes. A particularly important partner for ESA is the European Union, and the legal relationship between ESA and the EU will be explored in a separate article of this series. While science and engineering are at the heart of ESA’s mission, law is the backbone of ESA’s institutional structure: it enables compromise and cooperation, stability and flexibility, and planning and operation for an international organisation that is unique in many ways. More information can be found here: https://www.esa.int/About_Us/Law_at_ESA/Law_at_ESA.