This International Standard provides mathematical expressions for reliability, availability, maintainability and maintenance support measures defined in IEC 60050-192. According to this standard the dependability [192-01-22] is the ability of an item to perform as and when required and an item [192-01-01] may be an individual part, component, device, functional unit, equipment, subsystem, or system. Due to mathematical constraints, this standard splits the items between the individual items considered as a whole (e.g. individual components) and the systems made of several individual items. It provides general considerations about the mathematical expressions for systems as well individual items but the individual items which are easier to handle are analysed in more detail with regards to the repair aspects. The following item classes are considered separately: " Systems; " Individual items: non-repairable [192-01-12]; repairable [192-01-11]: " with zero (or negligible) time to restoration; " with non-zero time to restoration. In order to explain the dependability concepts which are not really easy to understand, keep the standard self-contained and the mathematical formulae as simple as possible, the following basic mathematical models are used to quantify dependability measures: " Systems: states-transitions models; Markovian models. " Individual items: random variable (time to failure) for non-repairable items; simple (ordinary) renewal process for repairable items with zero time to restoration; simple (ordinary) alternating renewal process for repairable items with non-zero time to restoration.