SpaceWire technology has grown from the needs of spacecraft on board data handling applications. This Standard provides a formal basis for the exploitation of SpaceWire in a wide range of future on board processing systems. One of the principal aims of SpaceWire is the support of equipment compatibility and reuse at both the component and subsystem levels. In principle a data handling system developed for an optical instrument, for example, can be used for a radar instrument by unplugging the optical sensor and plugging in the radar one. Processing units, mass memory units and down link telemetry systems developed for one mission can be readily used on another mission, reducing the cost of development, improving reliability and most importantly increasing the amount of scientific work that can be achieved within a limited budget. Integration and test of complex on board systems is also supported by SpaceWire with ground support equipment plugging directly into the on board data handling system. Monitoring and testing can be carried out with a seamless interface into the on board system. SpaceWire is the result of the efforts of many individuals within the European Space Agency, European Space Industry and academia. This standard may be tailored for the specific characteristics and constraints of a space project in conformance with ECSS-S-ST-00.