Abstract: Fault insertion circuits under programmable control and resident in an integrated circuit (LSI or VLSI) insert transient and intermittent fault classes in addition to a permanent fault class into functional logic on such integrated circuit. Specific fault types programmable for each fault class include a stuck-open fault and bridging faults both wired-AND and wired-OR. The programmable fault insertion circuitry on each integrated circuit interfaces directly or indirectly with a BIT maintenance controller. In addition to verifying test software, a fault tolerant system's error detection and recovery circuits may be verified by fault insertion testing using the transient and intermittent fault insertions.