Tunneling processes induced by terahertz frequency electric fields have been investigated. A drastic enhancement of the tunneling probability has been observed by increasing the frequency omega at omegatau(e) >> 1 where tau(e) is the tunneling time. For a given constant tunneling rate an increase of frequency by a factor of seven leads to a drop of the required electric field strength by three orders of magnitude. It is shown that the enhancement of tunneling ionization at terahertz frequencies is due to the fact that electrons can absorb energy from the radiation field during tunneling reducing the effective width of the tunneling barrier.