Fault tolerance is the ability of a system to continue to function normally in the face of one or more defects in one or more of it's components. When opposed to a naively constructed system, where even a minor failure might cause an entire breakdown, the drop in operating quality is proportional to the severity of the failure. In high-availability or life-critical systems, fault tolerance is especially important. Graceful degradation refers to a system's capacity to sustain functionality even when parts of it fail.Fault tolerance is the ability of a system to continue to function normally in the face of one or more defects in one or more of it's components. When opposed to a naively constructed system, where even a minor failure might cause an entire breakdown, the drop in operating quality is proportional to the severity of the failure. In high-availability or life-critical systems, fault tolerance is critical in the real world. Graceful degradation refers to a system's capacity to sustain functionality even when parts of it fail.

Antonn Svoboda constructed SAPO, the first known fault-tolerant computer, in Czechoslovakia in 1951. It's core architecture consisted of magnetic drums coupled with relays and a memory error detection system based on voting (triple modular redundancy). Several more machines were created along this line, especially for military use. They eventually split into three categories: machines that would last a long time without maintenance, such as those used on NASA space probes and satellites; computers that were very dependable but required constant monitoring, such as those used to monitor and control nuclear power plants or supercollider experiments; and computers with a large amount of runtime but would be under heavy use, such as many of the supercomputers used by insurances. Finally, computers with a significant runtime would be under heavy use, such as prime number finders.