Traditionally, availability has been seen as an atomic property asserting the average time a system is "up" or "down". In order to model and analyse the availability of computerised systems in a world where the dependency on and complexity of such systems are increasing, this notion of availability is no longer sufficient. This report presents a conceptual model for service availability designed to handle these challenges. The core of this model is a characterisation of service availability by means of accessibility properties and exclusivity properties, which is further specialised into measurable aspects of service availability. We outline how this conceptual model may be refined to a framework for specifying and analysing availability requirements.