Availability is defined as a system being available to authorized users when appropriate. Numerous threats can affect the availability of a system. The threat model should provide significant information as to what aspects of a system need availability protected and what aspects are not sensitive. Designing the best defenses against availability issues depends upon the threat to availability. Backups can provide a form of protection, as can data replication. Failover to redundant systems can also be employed. The key is in determining the specifics of the availability need and expressing them in a requirement that is then designed into the system.