This chapter reviews the various types of power line disturbances and their possible sources. Power quality disturbances such as voltage sags, surges, and impulses; faults in the shape of the voltage sine wave; high frequency noise; harmonics; and frequency errors can all cause intermittent failures of electronic loads. These same power disturbances can also shorten the expected life of an electronic device by causing increased heating and higher electrical stress. The end-user is responsible for providing an acceptable operating environment for the electronic load. Voltage variations are caused by unregulated utility feeders that experience changes in load over a period of time. Powerfails can cause the floating heads of disc drives to crash down on the disc, causing memory loss, unscheduled shutdown, or equipment damage. The chapter explores the categories of power line disturbances. Where a high degree of voltage regulation and total isolation from sags, dips, surges, and transients is required, an uninterruptible power supply may be used.