Ultrathin gate oxide, which is essential for low supply voltage and high driving capability, is indispensable four the continued scaling of ULSI technologies towards smaller and faster devices. Needless to say, the reliability of ultrathin oxide is of major concerns in the manufacturing of the state-of-the-art metal-oxide-semiconductor devices. This paper reviews the reliability issues regarding ultrathin gate oxide for present and future ULSI technologies. Issues including gate leakage current, time-dependent dielectric breakdown, poly-gate depletion, boron penetration, and plasma process-induced damage will be addressed. Several techniques such as nitrided oxide and alternative processes, which are proposed to improve gate oxide reliabilities, are also discussed.