The consideration of availability tracking and availability metrics at CERN differs significantly between different parts of the organisation. Equipment experts are predominantly concerned by equipment failure rate and repair time. Operators are concerned by the amount of time which is lost due to faults. Physicists are concerned by the amount of physics which is lost due to faults.
Over the course of the last 10 years CERN has developed several different approaches and metrics to deal with these different viewpoints. This paper will attempt to highlight some of these different approaches, and to explain the basic metrics that are used in colliders, such as the LHC, which differ to the concepts used in light sources.