It is a well-known fact that the exhaust emission characteristics of hydrogen fueled engines are extremely good. The external mixture formation - a hydrogen fuel supply method - has the merit of practically zero NOx emission level in the lean mixture range with the excess air ratio λ set at 2.0 or greater as well as the merits of simple mechanism and easy operation. However, the practical use of such engines has been impeded partly due to the occurrence of backfire where the excess air ratio λ is 2 to 3. In order to allow the practical use of the hydrogen fueled engines with external mixture formation, it is vital to determine the causes of backfire and to establish proper countermeasures.It is found through a recent study conducted on the mechanism of backfire that the abnormal electric discharge in the intake stroke is one of the causes of backfire. It is also found that the abnormal electric discharge is caused by the high energy retained in the ignition system due to low ion current flows resulting from lower ion concentrations in hydrogen flames relative to gasoline flames. The engine performance was improved dramatically by modifying the ignition system to prevent residual energy. However, backfire would still occur when the excess air ratio λ was 1.6 or so. The backfire threshold was improved by cooling the spark plug electrode with water to lower the temperature.