This Standard is part of a series of ECSS Standards belonging to ECSS Q ST-30, Space product assurance – Dependability. The present standard defines the requirements on availability activities and provides where necessary guidelines to support, plan and implement the activities. It defines the requirement typology that is followed, with regard to the availability of space systems or subsystems in order to meet the mission performance and needs according to the dependability and safety principles and objectives. This Standard also describes the process that is followed and the most significant methodologies for the availability analysis to cover such aspects as • evaluation of the space element or system availability figure, • allocation of the requirement at lower level, and • outputs to be provided. This Standard applies to all elements of a space project (flight and ground segments), where Availability analyses are part of the dependability programme, providing inputs for the system concept definition and design development. The onground activities and the operational phases are considered, for availability purposes, in order to • acquire additional information essential for a better system model finalization and evaluation, and • monitor the system behaviour to optimize its operational performance and improve the availability model for future applications. This standard may be tailored for the specific characteristic and constraints of a space project in conformance with ECSS-S-ST-00.