This paper will examine semiconductor wear out at San Onofre Nuclear Generation Station (SONGS). The topics will include case studies, failure mechanisms, diagnostic techniques, failure analysis techniques and root cause corrective actions. Nuclear power plants are unique in that instrumentation and control circuits are continuously energized, are periodically tested, and have been in operation for greater than 25 years. Root cause evaluations at SONGS have identified numerous semiconductor failures due to wear out. Case studies include light output deterioration in opto-isolators, junction alloying failures of transistors and integrated circuits and parametric shifts in operational amplifiers. In most cases the devices do not fail catastrophically but degraded to the point of circuit level functional failure. Failure analysis techniques include circuit analysis, board level troubleshooting to identify the degraded components. Intermittent failures require power cycling, thermal cycling, and long term monitoring to identify the responsible components. Corrective actions for semiconductor wear out at SONGS include enhanced monitoring and proactive change out of identified part types.