We review some experimental data on the dielectric breakdown (BD) phenomenon under high field stress of important gate insulators used in MOSFETs. We first discuss the case of silicon oxides or oxynitrides in the thickness range of tens of nm down to about 1 nm focusing on the kinetics of the final hard BD event. We describe the dependence of the BD current transient on the oxide thickness and the electric field and voltage. In particular we focus on the progressive BD phenomenon, ie, a gradual growth of the BD leakage occurring in a time scale which in operation conditions is relevant. We then discuss the BD phenomemon in the case of ultra-thin high polarizability constant (high-k) dielectrics with metal gates in comparison to the conventional SiO2/poly-Si devices and outline critical differences between these gate stacks.