The sources of electrical transients, broadly interpreted as occurrences of any disturbance, either on the power line or the computer system's data line, are reviewed. Common transients are overvoltages due to lightning strikes, transients caused by switching sequences in the power system, and undervoltages which could be caused by a nearby start-up of heavy loads or by distant faults. The impact of transients on small stand-alone and on distributed computer systems is examined. Growing concern among computer users that power-line surges, in particular, may damage equipment or cause loss of data has created a market for surge suppressors. Some guidelines are provided for choosing a protective device for both types of systems mentioned above. Potential negative side effects are indicated. >