The ignition system of any spark-ignited internal combustion engine-powered device is well recognized as a major source of radiated electromagnetic interference (EMI). Spark plug designs can be selected to aid in reducing ignition-related EMI. The effects of various spark plug designs such as resistor spark plugs, inductor spark plugs, and shielded spark plugs upon both EMI and ignition system performance are presented. In addition to studying the effect of spark plug design, the use of the SAE standard (J551b) and other EMI measurement techniques to determine effect of spark plug suppression designs are discussed. Generally, the addition of a resistor to the spark plug provides the most effective spark plug suppression. However, other designs may be effective as an addition to the resistor or as a substitute where resistors cannot be practically used.