This International Standard presents the requirements for Requirements Management (RM) for space projects. This International Standard addresses the space programme/project management requirements, applicable through a top-down approach in a contractual relationship between customers and suppliers. The objective of this International Standard is to state and establish a common reference framework for all the customers and suppliers in the space sector to deploy Requirements Management for all space products and projects. This International Standard on Requirements Management includes — a definition of the Requirements Management scope for the space sector, — the standard processes for Requirements Management within the product lifecycle management, and — a set of rules for Requirements Management activities to be implemented by the actors (customers and suppliers), including rules derived from best practices. The primary target audience for this International Standard includes — the Requirements Management/Systems Engineering process owners of the customers and suppliers, — the Programme/Project Managers managing the space programmes, and — the Chief Engineers and the Quality Managers. The term programme is understood as a group of several projects. Both “programme” and “project” may be used in the same context throughout this International Standard. ISO 21351 defines the requirements for the format and the content of the functional and technical specifications. In addition, it allows customer/supplier flexibility in its implementation and tailoring.