ABSTRACT
The entry introduces the concept of availability of information systems and information technology (IT). Starting from the definitions, it goes on to explain different ways to understand the concept, and how these have resulted in different scientific approaches over the years. Particular emphasis is given to the concept of service availability, and the corresponding notion of managing availability through Service Level Agreements. As IT service outages are very costly in modern society, the entry thoroughly introduces the economics of availability. The main formalisms for mathematical modeling of availability are also introduced.
