Experimental measurements and physical modeling of the tunneling current through ultra-thin gate oxides (1-6 nm) are presented for a large variety of experimental conditions including injection of electrons and holes from both accumulation and inversion layers and different cathode/anode polarities. By comparing experiments and simulations, the following issues are addressed. (i) importance of different components of the tunneling current; (ii) impact of quantization effects; (iii) sensitivity to the device structural parameters (doping levels, oxide thickness, etc.); (iv) impact of different stress conditions during aging experiments. The results of this study indicate the importance of quantum mechanical modeling and the presence of many tunneling mechanisms in ultra-thin oxide MOS devices, and that I-V measurements may be used for. oxide characterization at thickness levels where other techniques (C-V) become inaccurate.