measure of the continuous service accomplishment of the time to failure from a reference point 1. Compulsory misses caused by the first access to a block that has never been exd 2. Capacity misses 3. Conflict misses a cache miss that occurs because the cache, even with full associativity, cannot contain all the blocks 4. Fault avoidance prevent from occurring by design or construction 5. Fault tolerance redundancy to allow the service to continue despite faults from occurring 6. Reliability occurs when multiple block compete for the same location