This Standard defines a set of software product assurance requirements to be used for the development and maintenance of software for space systems. Space systems include manned and unmanned spacecraft, launchers, payloads, experiments and their associated ground equipment and facilities. Software includes the software component of firmware. This Standard also applies to the development or reuse of nondeliverable software which affects the quality of the deliverable product or service provided by a space system, if the service is implemented by software. ECSS-Q-ST-80 interfaces with space engineering and management, which are addressed in the Engineering (-E) and Management (-M) branches of the ECSS System, and explains how they relate to the software product assurance processes. This standard may be tailored for the specific characteristic and constrains of a space project in conformance with ECSS-S-ST-00. Tailoring of this Standard to a specific business agreement or project, when software product assurance requirements are prepared, is also addressed in clause 4.3.