High-frequency transients and impulses such as electrically fast transients (EFT) are caused by light switches, relay chatter, or motor start-up transients on the power line. These transients often occur in bursts and can cause upset to your product if the power line filtering is inadequate. The focus of this chapter will be with the IEC 61000-4-4 EFT test, but the concepts apply to all high-frequency transient events. The test is performed between line and neutral, line to safety ground, and neutral to safety ground and consists of a repeating burst of pulses. The test is also performed on I/O, signal, or data cables typically longer than 3 m (e.g., Ethernet) using a capacitive coupling fixture. Several performance levels may be acceptable (refer to the EFT standard IEC 61000-4-4 for details), but loss of data, system reset, or damage is generally considered a test failure.