Tunneling is fully quantum-mechanical effect that does not have classical analog. Tunneling has revolutionized surface science by its utilization in scanning tunneling microscopes. In some device applications tunneling is required for the operation of the device (Resonant tunneling diodes, EEPROMs – floating gate memories), but in some cases it leads to unwanted power dissipation, such as gate leakage in both MOS and Schottky transistors. Resonant tunneling diodes, due to the tunneling current at small biases exhibit negative differential resistance region and, thus, are suitable for oscillators. Because of this, it is very important to understand tunneling in double-barrier structure.